Messages in this thread | | | Date | Wed, 12 Jan 2022 15:06:12 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems |
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On Mon, Jan 10, 2022 at 11:41:23AM -0800, Tim Murray wrote:
> 1. f2fs-ckpt thread is running f2fs_write_checkpoint(), holding the > cp_rwsem write lock while doing so via f2fs_lock_all() in > block_operations(). > 2. Random very-low-priority thread A makes some other f2fs call that > tries to get the cp_rwsem read lock by atomically adding on the rwsem, > fails and deschedules in uninterruptible sleep. cp_rwsem now has a > non-zero reader count but is write-locked. > 3. f2fs-ckpt thread releases the cp_rwsem write lock. cp_rwsem now has > a non-zero reader count and is not write-locked, so is reader-locked. > 4. Other threads call fsync(), which requests checkpoints from > f2fs-ckpt, and block on a completion event that f2fs-ckpt dispatches. > cp_rwsem still has a non-zero reader count because the low-prio thread > A from (2) has not been scheduled again yet. > 5. f2fs-ckpt wakes up to perform checkpoints, but it stalls on the > write lock via cmpxchg in block_operations() until the low-prio thread > A has run and released the cp_rwsem read lock. Because f2fs-ckpt can't > run, all fsync() callers are also effectively blocked by the > low-priority thread holding the read lock. > > I think this is the rough shape of the problem (vs readers holding the > lock for too long or something like that) because the low-priority > thread is never run between when it is initially made runnable by > f2fs-ckpt and when it runs tens/hundreds of milliseconds later then > immediately unblocks f2fs-ckpt.
*urgh*... so you're making the worst case less likely but fundamentally you don't change anything.
If one of those low prio threads manages to block while holding cp_rwsem your checkpoint thread will still block for a very long time.
So while you improve the average case, the worst case doesn't improve much I think.
Also, given that this is a system wide rwsem, would percpu-rwsem not be 'better' ? Arguably with the same hack cgroups uses for it (see cgroup_init()) to lower the cost of percpu_down_write().
Now, I'm not a filesystem developer and I'm not much familiar with the problem space, but this locking reads like a fairly big problem. I'm not sure optimizing the lock is the answer.
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