| Message ID | 20210923170121.1860133-1-bigeasy@linutronix.de (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: Disable zsmalloc on PREEMPT_RT | expand |
On Thu, Sep 23, 2021 at 07:01:21PM +0200, Sebastian Andrzej Siewior wrote: > For efficiency reasons, zsmalloc is using a slim `handle'. The value is > the address of a memory allocation of 4 or 8 bytes depending on the size > of the long data type. The lowest bit in that allocated memory is used > as a bit spin lock. > The usage of the bit spin lock is problematic because with the bit spin > lock held zsmalloc acquires a rwlock_t and spinlock_t which are both > sleeping locks on PREEMPT_RT and therefore must not be acquired with > disabled preemption. I am not sure how long the preemption disabled section takes since it just disable for a page copy mostly. > > There is a patch which extends the handle on PREEMPT_RT so that a full > spinlock_t fits (even with lockdep enabled) and then eliminates the bit > spin lock. I'm not sure how sensible zsmalloc on PREEMPT_RT is given > that it is used to store compressed user memory. I don't see what's relation between PREEMPT_RT and compressed user memory so you can reach such conclustion. Disable zsmalloc also makes disable zram. I think in-compress memory swap rather than storage swap/block sometimes would be useful for even RT. > > Disable ZSMALLOC on PREEMPT_RT. If there is need for it, we can try to > get it to work. Please send the patch which extends handle with spin_lock rather than simply disabing. > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > mm/Kconfig | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/Kconfig b/mm/Kconfig > index 9f1e0098522c2..541371e64c477 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -640,6 +640,7 @@ config ZSWAP_ZPOOL_DEFAULT_Z3FOLD > > config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC > bool "zsmalloc" > + depends on !PREEMPT_RT > select ZSMALLOC > help > Use the zsmalloc allocator as the default allocator. > @@ -690,7 +691,7 @@ config Z3FOLD > > config ZSMALLOC > tristate "Memory allocator for compressed pages" > - depends on MMU > + depends on MMU && !PREEMPT_RT > help > zsmalloc is a slab-based memory allocator designed to store > compressed RAM pages. zsmalloc uses virtual memory mapping > -- > 2.33.0 > >
On 2021-09-23 16:06:58 [-0700], Minchan Kim wrote: > On Thu, Sep 23, 2021 at 07:01:21PM +0200, Sebastian Andrzej Siewior wrote: > > For efficiency reasons, zsmalloc is using a slim `handle'. The value is > > the address of a memory allocation of 4 or 8 bytes depending on the size > > of the long data type. The lowest bit in that allocated memory is used > > as a bit spin lock. > > The usage of the bit spin lock is problematic because with the bit spin > > lock held zsmalloc acquires a rwlock_t and spinlock_t which are both > > sleeping locks on PREEMPT_RT and therefore must not be acquired with > > disabled preemption. > > I am not sure how long the preemption disabled section takes since it > just disable for a page copy mostly. after the bit-spin-lock, there are sleeping locks. These are problematic. > > > > There is a patch which extends the handle on PREEMPT_RT so that a full > > spinlock_t fits (even with lockdep enabled) and then eliminates the bit > > spin lock. I'm not sure how sensible zsmalloc on PREEMPT_RT is given > > that it is used to store compressed user memory. > > I don't see what's relation between PREEMPT_RT and compressed user > memory so you can reach such conclustion. Disable zsmalloc also makes > disable zram. I think in-compress memory swap rather than storage > swap/block sometimes would be useful for even RT. The only user of zsmalloc I found was zswap which compressed user pages. Maybe I didn't collect all the dots. > > Disable ZSMALLOC on PREEMPT_RT. If there is need for it, we can try to > > get it to work. > > Please send the patch which extends handle with spin_lock rather than > simply disabing. Okay. I will clean it up a little and post it then. Sebastian
diff --git a/mm/Kconfig b/mm/Kconfig index 9f1e0098522c2..541371e64c477 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -640,6 +640,7 @@ config ZSWAP_ZPOOL_DEFAULT_Z3FOLD config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC bool "zsmalloc" + depends on !PREEMPT_RT select ZSMALLOC help Use the zsmalloc allocator as the default allocator. @@ -690,7 +691,7 @@ config Z3FOLD config ZSMALLOC tristate "Memory allocator for compressed pages" - depends on MMU + depends on MMU && !PREEMPT_RT help zsmalloc is a slab-based memory allocator designed to store compressed RAM pages. zsmalloc uses virtual memory mapping
For efficiency reasons, zsmalloc is using a slim `handle'. The value is the address of a memory allocation of 4 or 8 bytes depending on the size of the long data type. The lowest bit in that allocated memory is used as a bit spin lock. The usage of the bit spin lock is problematic because with the bit spin lock held zsmalloc acquires a rwlock_t and spinlock_t which are both sleeping locks on PREEMPT_RT and therefore must not be acquired with disabled preemption. There is a patch which extends the handle on PREEMPT_RT so that a full spinlock_t fits (even with lockdep enabled) and then eliminates the bit spin lock. I'm not sure how sensible zsmalloc on PREEMPT_RT is given that it is used to store compressed user memory. Disable ZSMALLOC on PREEMPT_RT. If there is need for it, we can try to get it to work. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- mm/Kconfig | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)