| Message ID | 20171019074705.24827-1-hch@lst.de (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
On Thu, Oct 19, 2017 at 09:47:05AM +0200, Christoph Hellwig wrote: > Apparently our current rwsem code doesn't like doing the trylock, then > lock for real scheme. So change our read/write methods to just do the > trylock for the RWF_NOWAIT case. This fixes a ~25% regression in > AIM7. > The code looks fine, but this seems really strange. If the trylock fails, then wouldn't the blocking lock have slept anyways if done initially? Is there any more background info available on this, or perhaps a theory on why there is such a significant regression..? Brian > Fixes: 91f9943e ("fs: support RWF_NOWAIT for buffered reads") > Reported-by: kernel test robot <xiaolong.ye@intel.com> > Signed-off-by: Christoph Hellwig <hch@lst.de> > --- > fs/xfs/xfs_file.c | 21 +++++++++++++-------- > 1 file changed, 13 insertions(+), 8 deletions(-) > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c > index 309e26c9dddb..f40b5da5d467 100644 > --- a/fs/xfs/xfs_file.c > +++ b/fs/xfs/xfs_file.c > @@ -237,11 +237,13 @@ xfs_file_dax_read( > if (!count) > return 0; /* skip atime */ > > - if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) { > - if (iocb->ki_flags & IOCB_NOWAIT) > + if (iocb->ki_flags & IOCB_NOWAIT) { > + if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) > return -EAGAIN; > + } else { > xfs_ilock(ip, XFS_IOLOCK_SHARED); > } > + > ret = dax_iomap_rw(iocb, to, &xfs_iomap_ops); > xfs_iunlock(ip, XFS_IOLOCK_SHARED); > > @@ -259,9 +261,10 @@ xfs_file_buffered_aio_read( > > trace_xfs_file_buffered_read(ip, iov_iter_count(to), iocb->ki_pos); > > - if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) { > - if (iocb->ki_flags & IOCB_NOWAIT) > + if (iocb->ki_flags & IOCB_NOWAIT) { > + if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) > return -EAGAIN; > + } else { > xfs_ilock(ip, XFS_IOLOCK_SHARED); > } > ret = generic_file_read_iter(iocb, to); > @@ -552,9 +555,10 @@ xfs_file_dio_aio_write( > iolock = XFS_IOLOCK_SHARED; > } > > - if (!xfs_ilock_nowait(ip, iolock)) { > - if (iocb->ki_flags & IOCB_NOWAIT) > + if (iocb->ki_flags & IOCB_NOWAIT) { > + if (!xfs_ilock_nowait(ip, iolock)) > return -EAGAIN; > + } else { > xfs_ilock(ip, iolock); > } > > @@ -606,9 +610,10 @@ xfs_file_dax_write( > size_t count; > loff_t pos; > > - if (!xfs_ilock_nowait(ip, iolock)) { > - if (iocb->ki_flags & IOCB_NOWAIT) > + if (iocb->ki_flags & IOCB_NOWAIT) { > + if (!xfs_ilock_nowait(ip, iolock)) > return -EAGAIN; > + } else { > xfs_ilock(ip, iolock); > } > > -- > 2.14.2 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Oct 19, 2017 at 07:38:48AM -0400, Brian Foster wrote: > On Thu, Oct 19, 2017 at 09:47:05AM +0200, Christoph Hellwig wrote: > > Apparently our current rwsem code doesn't like doing the trylock, then > > lock for real scheme. So change our read/write methods to just do the > > trylock for the RWF_NOWAIT case. This fixes a ~25% regression in > > AIM7. > > > > The code looks fine, but this seems really strange. If the trylock > fails, then wouldn't the blocking lock have slept anyways if done > initially? Is there any more background info available on this, or > perhaps a theory on why there is such a significant regression..? No, unfortunately I don't have a theory, but I agree it is odd behavior in the rwsem code. -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Oct 19, 2017 at 03:14:07PM +0200, Christoph Hellwig wrote: > On Thu, Oct 19, 2017 at 07:38:48AM -0400, Brian Foster wrote: > > On Thu, Oct 19, 2017 at 09:47:05AM +0200, Christoph Hellwig wrote: > > > Apparently our current rwsem code doesn't like doing the trylock, then > > > lock for real scheme. So change our read/write methods to just do the > > > trylock for the RWF_NOWAIT case. This fixes a ~25% regression in > > > AIM7. > > > > > > > The code looks fine, but this seems really strange. If the trylock > > fails, then wouldn't the blocking lock have slept anyways if done > > initially? Is there any more background info available on this, or > > perhaps a theory on why there is such a significant regression..? > > No, unfortunately I don't have a theory, but I agree it is odd > behavior in the rwsem code. <shrug> I want to know a little more about why there's a performance hit in the down_read_trylock -> down_read case. Are we getting penalized for that? Is it some weird interaction with lockdep? --D > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Oct 19, 2017 at 03:44:31PM -0700, Darrick J. Wong wrote: > > > The code looks fine, but this seems really strange. If the trylock > > > fails, then wouldn't the blocking lock have slept anyways if done > > > initially? Is there any more background info available on this, or > > > perhaps a theory on why there is such a significant regression..? > > > > No, unfortunately I don't have a theory, but I agree it is odd > > behavior in the rwsem code. > > <shrug> I want to know a little more about why there's a performance hit > in the down_read_trylock -> down_read case. Are we getting penalized > for that? Is it some weird interaction with lockdep? I don't think the test bot did run with lockdep. But feel free to take a look at the mail thread titled [lkp-robot] [fs] 91f9943e1c: aim7.jobs-per-min -26.6% regression on lkml. Note that synthetic benchmarks on XFS always saw weird effects from rwsem details. I remember that a few years ago I had to back to the mainline patch to move the rwsem fastpath out of line because thay caused a major performance regressions on CIFS file serving benchmarks on a very low end ARM NAS box. -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu 19-10-17 15:44:31, Darrick J. Wong wrote: > On Thu, Oct 19, 2017 at 03:14:07PM +0200, Christoph Hellwig wrote: > > On Thu, Oct 19, 2017 at 07:38:48AM -0400, Brian Foster wrote: > > > On Thu, Oct 19, 2017 at 09:47:05AM +0200, Christoph Hellwig wrote: > > > > Apparently our current rwsem code doesn't like doing the trylock, then > > > > lock for real scheme. So change our read/write methods to just do the > > > > trylock for the RWF_NOWAIT case. This fixes a ~25% regression in > > > > AIM7. > > > > > > > > > > The code looks fine, but this seems really strange. If the trylock > > > fails, then wouldn't the blocking lock have slept anyways if done > > > initially? Is there any more background info available on this, or > > > perhaps a theory on why there is such a significant regression..? > > > > No, unfortunately I don't have a theory, but I agree it is odd > > behavior in the rwsem code. > > <shrug> I want to know a little more about why there's a performance hit > in the down_read_trylock -> down_read case. Are we getting penalized > for that? Is it some weird interaction with lockdep? At least on x86, __down_read_trylock() is implemented very much differently from __down_read(). In particular if there's heavy contention on the semaphore from readers, __down_read_trylock() implementation seems to be prone to going through cmpxchg loop several times which could explain observed performance data. But I'm just guessing... Adding some x86 people to CC just in case they have more to say. Honza
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index 309e26c9dddb..f40b5da5d467 100644 --- a/fs/xfs/xfs_file.c +++ b/fs/xfs/xfs_file.c @@ -237,11 +237,13 @@ xfs_file_dax_read( if (!count) return 0; /* skip atime */ - if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) { - if (iocb->ki_flags & IOCB_NOWAIT) + if (iocb->ki_flags & IOCB_NOWAIT) { + if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) return -EAGAIN; + } else { xfs_ilock(ip, XFS_IOLOCK_SHARED); } + ret = dax_iomap_rw(iocb, to, &xfs_iomap_ops); xfs_iunlock(ip, XFS_IOLOCK_SHARED); @@ -259,9 +261,10 @@ xfs_file_buffered_aio_read( trace_xfs_file_buffered_read(ip, iov_iter_count(to), iocb->ki_pos); - if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) { - if (iocb->ki_flags & IOCB_NOWAIT) + if (iocb->ki_flags & IOCB_NOWAIT) { + if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) return -EAGAIN; + } else { xfs_ilock(ip, XFS_IOLOCK_SHARED); } ret = generic_file_read_iter(iocb, to); @@ -552,9 +555,10 @@ xfs_file_dio_aio_write( iolock = XFS_IOLOCK_SHARED; } - if (!xfs_ilock_nowait(ip, iolock)) { - if (iocb->ki_flags & IOCB_NOWAIT) + if (iocb->ki_flags & IOCB_NOWAIT) { + if (!xfs_ilock_nowait(ip, iolock)) return -EAGAIN; + } else { xfs_ilock(ip, iolock); } @@ -606,9 +610,10 @@ xfs_file_dax_write( size_t count; loff_t pos; - if (!xfs_ilock_nowait(ip, iolock)) { - if (iocb->ki_flags & IOCB_NOWAIT) + if (iocb->ki_flags & IOCB_NOWAIT) { + if (!xfs_ilock_nowait(ip, iolock)) return -EAGAIN; + } else { xfs_ilock(ip, iolock); }
Apparently our current rwsem code doesn't like doing the trylock, then lock for real scheme. So change our read/write methods to just do the trylock for the RWF_NOWAIT case. This fixes a ~25% regression in AIM7. Fixes: 91f9943e ("fs: support RWF_NOWAIT for buffered reads") Reported-by: kernel test robot <xiaolong.ye@intel.com> Signed-off-by: Christoph Hellwig <hch@lst.de> --- fs/xfs/xfs_file.c | 21 +++++++++++++-------- 1 file changed, 13 insertions(+), 8 deletions(-)