Re: [PATCH v2] squashfs: avoid thundering-herd cache wakeups

[Phillip Lougher <phillip@squashfs.org.uk>]

On 07/08/2026 18:24, Usama Arif wrote:
> squashfs_cache_get() puts a task to sleep when its block is not cached
> and every cache entry is busy.  Those sleeps are non-exclusive, so the
> nr_exclusive == 1 budget squashfs_cache_put() has always passed to
> wake_up() is inert and one release makes every waiter runnable.  A wakee
> only returns to squashfs_cache_get() if it observes cache->unused before
> the entry is reclaimed; later wakees see zero and re-queue inside
> wait_event() without rescanning.  One freed entry satisfies exactly one
> capacity waiter, so waking the rest is waste.
> 
> On a Meta production host serving a Python web application from a
> packaged squashfs image, a 30-second trace caught 1,045,132
> cache-release wake calls and 19,511,556 wakeups: 18.7 per release,
> although each release added only one reusable cache entry.  This was
> causing significant spikes in CPU usage.
> 
> Make the waits exclusive, enqueueing while still holding cache->lock so
> that a concurrent lookup either sees the waiter queued or the waiter
> sees the block that lookup publishes.  Two things follow.
> 
> A wakee cannot be assumed to consume the entry it was woken for: it may
> find its own block published meanwhile, share that entry, and leave the
> freed one unclaimed.  So a wakee which shares hands its wakeup on to the
> next waiter, as commit 0ddad21d3e99 ("pipe: use exclusive waits when
> reading or writing") does with wake_next_reader.
> 
> And a waiter can now sleep through a publication of the very block it
> wants, which the old broadcast gave it repeated chances to notice.  So
> waiters are keyed by block: publishing wakes every waiter for that block
> (nr_exclusive == 0), freeing an entry wakes one.  That needs a custom
> wake callback, like wake_page_function() in mm/filemap.c, which also
> records which wakeup arrived so the handoff only fires for a capacity
> wakee.
> 
> Broadcast is kept where more than one task can proceed - every waiter
> for a published block, and the wake_up_all() on entry->wait_queue - at
> the cost of walking the queue under wait_queue.lock to test the key.
> Waiters are now served FIFO with a scheduling round trip per handoff
> hop, so per-waiter latency changes; the filebench run below is 4x
> oversubscribed, where that should hurt most.
> 
> Measured on a 32-CPU VM against a read-only squashfs (gzip,
> DECOMP_MULTI_PERCPU, FILE_DIRECT, default 8 metadata / 3 fragment cache
> entries) staged in tmpfs, page cache dropped each iteration to force
> cold decompression:
> 
>    elbencho, 64 threads
>      metadata stat        700 ->  1320 files/s    1.9x
>      small-file read       40 ->    60 MiB/s      1.5x
> 
>    filebench, 128 threads, open+read+stat+close (mean of 3x 30s)
>      throughput        11,314 -> 25,186 ops/s     2.2x
>      sched:sched_wakeup  27.0 ->   4.55 per op    5.9x fewer
>      context switches    37.2 ->   7.64 per op    4.9x fewer
> 
> Wakeups and context switches are per operation, since the two runs did
> 2.2x different amounts of work.  Workloads which never queue for a cache
> entry gain no wakeups.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>   fs/squashfs/cache.c          | 114 +++++++++++++++++++++++++++++++++--
>   fs/squashfs/squashfs_fs_sb.h |   9 +++
>   2 files changed, 117 insertions(+), 6 deletions(-)

Very nice performance improvement.  Thanks.

Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>