| Message ID | 20210812084148.14458-3-hreitz@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block-status cache for data regions | expand |
On Thu, Aug 12, 2021 at 10:41:44AM +0200, Hanna Reitz wrote: > As we have attempted before > (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, > "file-posix: Cache lseek result for data regions"; > https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, > "file-posix: Cache next hole"), this patch seeks to reduce the number of > SEEK_DATA/HOLE operations the file-posix driver has to perform. The > main difference is that this time it is implemented as part of the > general block layer code. > > We only use the cache for nodes without children (i.e. protocol nodes), > because that is where the problem is: Drivers that rely on block-status > implementations outside of qemu (e.g. SEEK_DATA/HOLE). > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 > Signed-off-by: Hanna Reitz <hreitz@redhat.com> > --- > +++ b/block.c > +/** > + * Check whether [offset, offset + bytes) overlaps with the cached > + * block-status data region. > + * > + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, > + * which is what bdrv_bsc_is_data()'s interface needs. > + * Otherwise, *pnum is not touched. Why duplicate this comment,... > + */ > +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, > + int64_t offset, int64_t bytes, > + int64_t *pnum) > +{ > + BdrvBlockStatusCache *bsc = qatomic_rcu_read(&bs->block_status_cache); > + bool overlaps; > + > + overlaps = > + qatomic_read(&bsc->valid) && > + ranges_overlap(offset, bytes, bsc->data_start, > + bsc->data_end - bsc->data_start); > + > + if (overlaps && pnum) { > + *pnum = bsc->data_end - offset; > + } > + > + return overlaps; > +} > + > +/** > + * See block_int.h for this function's documentation. > + */ > +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum) > +{ > + RCU_READ_LOCK_GUARD(); > + > + return bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); > +} > + > +/** > + * See block_int.h for this function's documentation. ...but not these? But that's minor. Reviewed-by: Eric Blake <eblake@redhat.com>
On 16.08.21 23:38, Eric Blake wrote: > On Thu, Aug 12, 2021 at 10:41:44AM +0200, Hanna Reitz wrote: >> As we have attempted before >> (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, >> "file-posix: Cache lseek result for data regions"; >> https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, >> "file-posix: Cache next hole"), this patch seeks to reduce the number of >> SEEK_DATA/HOLE operations the file-posix driver has to perform. The >> main difference is that this time it is implemented as part of the >> general block layer code. >> >> We only use the cache for nodes without children (i.e. protocol nodes), >> because that is where the problem is: Drivers that rely on block-status >> implementations outside of qemu (e.g. SEEK_DATA/HOLE). >> >> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 >> Signed-off-by: Hanna Reitz <hreitz@redhat.com> >> --- >> +++ b/block.c >> +/** >> + * Check whether [offset, offset + bytes) overlaps with the cached >> + * block-status data region. >> + * >> + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, >> + * which is what bdrv_bsc_is_data()'s interface needs. >> + * Otherwise, *pnum is not touched. > Why duplicate this comment,... I don’t think it can be duplicated, because this is a static function. It is very similar to bdrv_bsc_is_data()’s interface, I admit, but it isn’t exactly the same (besides the _locked suffix, the only difference is that bdrv_bsc_is_data() is for a single byte, while this function checks a range). >> + */ >> +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, >> + int64_t offset, int64_t bytes, >> + int64_t *pnum) >> +{ >> + BdrvBlockStatusCache *bsc = qatomic_rcu_read(&bs->block_status_cache); >> + bool overlaps; >> + >> + overlaps = >> + qatomic_read(&bsc->valid) && >> + ranges_overlap(offset, bytes, bsc->data_start, >> + bsc->data_end - bsc->data_start); >> + >> + if (overlaps && pnum) { >> + *pnum = bsc->data_end - offset; >> + } >> + >> + return overlaps; >> +} >> + >> +/** >> + * See block_int.h for this function's documentation. >> + */ >> +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum) >> +{ >> + RCU_READ_LOCK_GUARD(); >> + >> + return bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); >> +} >> + >> +/** >> + * See block_int.h for this function's documentation. > ...but not these? These are not static functions, and so I can point to the header where they’re declared. (We have a wild mix of where functions are described in qemu, and it’s often in their C files. I prefer having descriptions in the header, but because we have the precedent of explaining interfaces in C files, I thought I can’t get around adding at least pointers in the C file.) > But that's minor. > > Reviewed-by: Eric Blake <eblake@redhat.com> Thanks! Hanna
12.08.2021 11:41, Hanna Reitz wrote: > As we have attempted before > (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, > "file-posix: Cache lseek result for data regions"; > https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, > "file-posix: Cache next hole"), this patch seeks to reduce the number of > SEEK_DATA/HOLE operations the file-posix driver has to perform. The > main difference is that this time it is implemented as part of the > general block layer code. > > The problem we face is that on some filesystems or in some > circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the > implementation is outside of qemu, there is little we can do about its > performance. > > We have already introduced the want_zero parameter to > bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls > unless we really want zero information; but sometimes we do want that > information, because for files that consist largely of zero areas, > special-casing those areas can give large performance boosts. So the > real problem is with files that consist largely of data, so that > inquiring the block status does not gain us much performance, but where > such an inquiry itself takes a lot of time. > > To address this, we want to cache data regions. Most of the time, when > bad performance is reported, it is in places where the image is iterated > over from start to end (qemu-img convert or the mirror job), so a simple > yet effective solution is to cache only the current data region. > > (Note that only caching data regions but not zero regions means that > returning false information from the cache is not catastrophic: Treating > zeroes as data is fine. While we try to invalidate the cache on zero > writes and discards, such incongruences may still occur when there are > other processes writing to the image.) > > We only use the cache for nodes without children (i.e. protocol nodes), > because that is where the problem is: Drivers that rely on block-status > implementations outside of qemu (e.g. SEEK_DATA/HOLE). > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 > Signed-off-by: Hanna Reitz <hreitz@redhat.com> > --- > include/block/block_int.h | 50 ++++++++++++++++++++++++ [..] > + /* > + * Note that checking QLIST_EMPTY(&bs->children) is also done when > + * the cache is queried above. Technically, we do not need to check > + * it here; the worst that can happen is that we fill the cache for > + * non-protocol nodes, and then it is never used. However, filling > + * the cache requires an RCU update, so double check here to avoid > + * such an update if possible. > + */ > + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && > + QLIST_EMPTY(&bs->children)) > + { > + /* > + * When a protocol driver reports BLOCK_OFFSET_VALID, the > + * returned local_map value must be the same as the offset we > + * have passed (aligned_offset). > + * Assert this, because we follow this rule when reading from > + * the cache (see the `local_map = aligned_offset` assignment > + * above), and the result the cache delivers must be the same > + * as the driver would deliver. > + */ > + assert(local_map == aligned_offset); maybe, also assert(local_file == bs); as well, we may check only BDRV_BLOCK_DATA flag above and assert BDRV_BLOCK_OFFSET_VALID here anyway: Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
diff --git a/include/block/block_int.h b/include/block/block_int.h index 12e5750fe8..437d746733 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -34,6 +34,7 @@ #include "qemu/hbitmap.h" #include "block/snapshot.h" #include "qemu/throttle.h" +#include "qemu/rcu.h" #define BLOCK_FLAG_LAZY_REFCOUNTS 8 @@ -839,6 +840,24 @@ struct BdrvChild { QLIST_ENTRY(BdrvChild) next_parent; }; +/* + * Allows bdrv_co_block_status() to cache one data region for a + * protocol node. + * + * @valid: Whether the cache is valid (should be accessed with atomic + * functions so this can be reset by RCU readers) + * @data_start: Offset where we know (or strongly assume) is data + * @data_end: Offset where the data region ends (which is not necessarily + * the start of a zeroed region) + */ +typedef struct BdrvBlockStatusCache { + struct rcu_head rcu; + + bool valid; + int64_t data_start; + int64_t data_end; +} BdrvBlockStatusCache; + struct BlockDriverState { /* Protected by big QEMU lock or read-only after opening. No special * locking needed during I/O... @@ -1004,6 +1023,11 @@ struct BlockDriverState { /* BdrvChild links to this node may never be frozen */ bool never_freeze; + + /* Lock for block-status cache RCU writers */ + CoMutex bsc_modify_lock; + /* Always non-NULL, but must only be dereferenced under an RCU read guard */ + BdrvBlockStatusCache *block_status_cache; }; struct BlockBackendRootState { @@ -1429,4 +1453,30 @@ static inline BlockDriverState *bdrv_primary_bs(BlockDriverState *bs) */ void bdrv_drain_all_end_quiesce(BlockDriverState *bs); +/** + * Check whether the given offset is in the cached block-status data + * region. + * + * If it is, and @pnum is not NULL, *pnum is set to + * `bsc.data_end - offset`, i.e. how many bytes, starting from + * @offset, are data (according to the cache). + * Otherwise, *pnum is not touched. + */ +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum); + +/** + * If [offset, offset + bytes) overlaps with the currently cached + * block-status region, invalidate the cache. + * + * (To be used by I/O paths that cause data regions to be zero or + * holes.) + */ +void bdrv_bsc_invalidate_range(BlockDriverState *bs, + int64_t offset, int64_t bytes); + +/** + * Mark the range [offset, offset + bytes) as a data region. + */ +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes); + #endif /* BLOCK_INT_H */ diff --git a/block.c b/block.c index e97ce0b1c8..28b00d7239 100644 --- a/block.c +++ b/block.c @@ -49,6 +49,8 @@ #include "qemu/timer.h" #include "qemu/cutils.h" #include "qemu/id.h" +#include "qemu/range.h" +#include "qemu/rcu.h" #include "block/coroutines.h" #ifdef CONFIG_BSD @@ -401,6 +403,9 @@ BlockDriverState *bdrv_new(void) qemu_co_queue_init(&bs->flush_queue); + qemu_co_mutex_init(&bs->bsc_modify_lock); + bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1); + for (i = 0; i < bdrv_drain_all_count; i++) { bdrv_drained_begin(bs); } @@ -4694,6 +4699,8 @@ static void bdrv_close(BlockDriverState *bs) bs->explicit_options = NULL; qobject_unref(bs->full_open_options); bs->full_open_options = NULL; + g_free(bs->block_status_cache); + bs->block_status_cache = NULL; bdrv_release_named_dirty_bitmaps(bs); assert(QLIST_EMPTY(&bs->dirty_bitmaps)); @@ -7653,3 +7660,76 @@ BlockDriverState *bdrv_backing_chain_next(BlockDriverState *bs) { return bdrv_skip_filters(bdrv_cow_bs(bdrv_skip_filters(bs))); } + +/** + * Check whether [offset, offset + bytes) overlaps with the cached + * block-status data region. + * + * If so, and @pnum is not NULL, set *pnum to `bsc.data_end - offset`, + * which is what bdrv_bsc_is_data()'s interface needs. + * Otherwise, *pnum is not touched. + */ +static bool bdrv_bsc_range_overlaps_locked(BlockDriverState *bs, + int64_t offset, int64_t bytes, + int64_t *pnum) +{ + BdrvBlockStatusCache *bsc = qatomic_rcu_read(&bs->block_status_cache); + bool overlaps; + + overlaps = + qatomic_read(&bsc->valid) && + ranges_overlap(offset, bytes, bsc->data_start, + bsc->data_end - bsc->data_start); + + if (overlaps && pnum) { + *pnum = bsc->data_end - offset; + } + + return overlaps; +} + +/** + * See block_int.h for this function's documentation. + */ +bool bdrv_bsc_is_data(BlockDriverState *bs, int64_t offset, int64_t *pnum) +{ + RCU_READ_LOCK_GUARD(); + + return bdrv_bsc_range_overlaps_locked(bs, offset, 1, pnum); +} + +/** + * See block_int.h for this function's documentation. + */ +void bdrv_bsc_invalidate_range(BlockDriverState *bs, + int64_t offset, int64_t bytes) +{ + RCU_READ_LOCK_GUARD(); + + if (bdrv_bsc_range_overlaps_locked(bs, offset, bytes, NULL)) { + qatomic_set(&bs->block_status_cache->valid, false); + } +} + +/** + * See block_int.h for this function's documentation. + */ +void bdrv_bsc_fill(BlockDriverState *bs, int64_t offset, int64_t bytes) +{ + BdrvBlockStatusCache *new_bsc = g_new(BdrvBlockStatusCache, 1); + BdrvBlockStatusCache *old_bsc; + + *new_bsc = (BdrvBlockStatusCache) { + .valid = true, + .data_start = offset, + .data_end = offset + bytes, + }; + + QEMU_LOCK_GUARD(&bs->bsc_modify_lock); + + old_bsc = qatomic_rcu_read(&bs->block_status_cache); + qatomic_rcu_set(&bs->block_status_cache, new_bsc); + if (old_bsc) { + g_free_rcu(old_bsc, rcu); + } +} diff --git a/block/io.c b/block/io.c index a19942718b..e62cdbda99 100644 --- a/block/io.c +++ b/block/io.c @@ -1883,6 +1883,9 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, return -ENOTSUP; } + /* Invalidate the cached block-status data range if this write overlaps */ + bdrv_bsc_invalidate_range(bs, offset, bytes); + assert(alignment % bs->bl.request_alignment == 0); head = offset % alignment; tail = (offset + bytes) % alignment; @@ -2447,9 +2450,62 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs, aligned_bytes = ROUND_UP(offset + bytes, align) - aligned_offset; if (bs->drv->bdrv_co_block_status) { - ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, - aligned_bytes, pnum, &local_map, - &local_file); + /* + * Use the block-status cache only for protocol nodes: Format + * drivers are generally quick to inquire the status, but protocol + * drivers often need to get information from outside of qemu, so + * we do not have control over the actual implementation. There + * have been cases where inquiring the status took an unreasonably + * long time, and we can do nothing in qemu to fix it. + * This is especially problematic for images with large data areas, + * because finding the few holes in them and giving them special + * treatment does not gain much performance. Therefore, we try to + * cache the last-identified data region. + * + * Second, limiting ourselves to protocol nodes allows us to assume + * the block status for data regions to be DATA | OFFSET_VALID, and + * that the host offset is the same as the guest offset. + * + * Note that it is possible that external writers zero parts of + * the cached regions without the cache being invalidated, and so + * we may report zeroes as data. This is not catastrophic, + * however, because reporting zeroes as data is fine. + */ + if (QLIST_EMPTY(&bs->children) && + bdrv_bsc_is_data(bs, aligned_offset, pnum)) + { + ret = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID; + local_file = bs; + local_map = aligned_offset; + } else { + ret = bs->drv->bdrv_co_block_status(bs, want_zero, aligned_offset, + aligned_bytes, pnum, &local_map, + &local_file); + + /* + * Note that checking QLIST_EMPTY(&bs->children) is also done when + * the cache is queried above. Technically, we do not need to check + * it here; the worst that can happen is that we fill the cache for + * non-protocol nodes, and then it is never used. However, filling + * the cache requires an RCU update, so double check here to avoid + * such an update if possible. + */ + if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) && + QLIST_EMPTY(&bs->children)) + { + /* + * When a protocol driver reports BLOCK_OFFSET_VALID, the + * returned local_map value must be the same as the offset we + * have passed (aligned_offset). + * Assert this, because we follow this rule when reading from + * the cache (see the `local_map = aligned_offset` assignment + * above), and the result the cache delivers must be the same + * as the driver would deliver. + */ + assert(local_map == aligned_offset); + bdrv_bsc_fill(bs, aligned_offset, *pnum); + } + } } else { /* Default code for filters */ @@ -3002,6 +3058,9 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset, return 0; } + /* Invalidate the cached block-status data range if this discard overlaps */ + bdrv_bsc_invalidate_range(bs, offset, bytes); + /* Discard is advisory, but some devices track and coalesce * unaligned requests, so we must pass everything down rather than * round here. Still, most devices will just silently ignore
As we have attempted before (https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html, "file-posix: Cache lseek result for data regions"; https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html, "file-posix: Cache next hole"), this patch seeks to reduce the number of SEEK_DATA/HOLE operations the file-posix driver has to perform. The main difference is that this time it is implemented as part of the general block layer code. The problem we face is that on some filesystems or in some circumstances, SEEK_DATA/HOLE is unreasonably slow. Given the implementation is outside of qemu, there is little we can do about its performance. We have already introduced the want_zero parameter to bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls unless we really want zero information; but sometimes we do want that information, because for files that consist largely of zero areas, special-casing those areas can give large performance boosts. So the real problem is with files that consist largely of data, so that inquiring the block status does not gain us much performance, but where such an inquiry itself takes a lot of time. To address this, we want to cache data regions. Most of the time, when bad performance is reported, it is in places where the image is iterated over from start to end (qemu-img convert or the mirror job), so a simple yet effective solution is to cache only the current data region. (Note that only caching data regions but not zero regions means that returning false information from the cache is not catastrophic: Treating zeroes as data is fine. While we try to invalidate the cache on zero writes and discards, such incongruences may still occur when there are other processes writing to the image.) We only use the cache for nodes without children (i.e. protocol nodes), because that is where the problem is: Drivers that rely on block-status implementations outside of qemu (e.g. SEEK_DATA/HOLE). Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307 Signed-off-by: Hanna Reitz <hreitz@redhat.com> --- include/block/block_int.h | 50 ++++++++++++++++++++++++ block.c | 80 +++++++++++++++++++++++++++++++++++++++ block/io.c | 65 +++++++++++++++++++++++++++++-- 3 files changed, 192 insertions(+), 3 deletions(-)