fs/pipe: reduce pipe->mutex contention by pre-allocating outside the lock#1445
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fs/pipe: reduce pipe->mutex contention by pre-allocating outside the lock#1445vfsci-bot[bot] wants to merge 2 commits into
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anon_pipe_write() takes pipe->mutex (aka "mutex protecting the whole thing") and then, from the per-iteration anon_pipe_get_page() helper, used to call alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT) once per page while still holding it. That allocation can sleep doing direct reclaim and/or runs memcg charging, which extends the critical section and stalls a concurrent reader on the very same mutex. Just pre-alloc the required pages before the lock in an array and just pop them inside the lock. This can improve the pipe throughput up to 48% and reduce the latency in 33%, easily seen when there is memory pressure and direct reclaim. Signed-off-by: Breno Leitao <leitao@debian.org>
Add a small selftest that stresses pipe->mutex contention by spawning N writer threads that hammer a single pipe with multi-page writes, plus M reader threads that drain. Each writer records its own write() latency samples into a log2-bucketed histogram; main aggregates and prints total writes, throughput, average and percentile (p50/p99) latencies, and the maximum observed latency. Pass --memory-pressure to fork stress-ng (--vm 4 --vm-bytes 80% --vm-method all) for the duration of the run, so alloc_page() in anon_pipe_write() routinely hits direct reclaim. The flag fails fast if stress-ng is not on $PATH. Program print something like the following, for different writes, readers, msgsizes and memory pressure: config: writers=X readers=Y msgsize=Z duration=3 pipe_size=1048576 memory_pressure=[no|yes] writes: total=54451 rate=18150/s throughput_MBps: 1134.40 lat_avg_ns: 275355 lat_p50_ns_upper: 262143 lat_p99_ns_upper: 1048575 lat_max_ns: 2145633 Signed-off-by: Breno Leitao <leitao@debian.org>
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Series: https://patchwork.kernel.org/project/linux-fsdevel/list/?series=1099509
Submitter: Breno Leitao
Version: 2
Patches: 2/2
Message-ID:
<20260522-fix_pipe-v2-0-a8b35a78244e@debian.org>Base: vfs.base.ci
Lore: https://lore.kernel.org/linux-fsdevel/20260522-fix_pipe-v2-0-a8b35a78244e@debian.org
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