From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E74733655F1; Fri, 22 May 2026 19:43:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779479031; cv=none; b=avT+WeiOH9mshEo9mRD8ddvDkj/kqQSTBvIgRbJ+QR9BNXtNk+sjDAE5Xmbeny7aMlafLbKVtOpYWDd93GCHNIwOOK0qwgzVkRWtkeEHnvDpxYulXliFWf6srHibRtTgrXGglA9uv0xtVgbwNR2YfRMg6UNedQEfivtNs4+iqVg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779479031; c=relaxed/simple; bh=aQrscosaX/SWgi2yUuQVWGozRU+m/4b3LcYPuaOg8H8=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=RRzOPa0Zhu4z84tj/5QZWAZecH7QazZqhvz6vWx1XKGfd2Czg0xojwKjqtoBAkpvSu9yVP+AhxyV6lXNykOFu+F6yCFvV3J8MiAdoFz1R0OVtvobavXKKRhxzooZbM+B2xDo6F6E1/oOUh7VA4am06OYl58n8jVT7yUfNfdtdws= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=H4+SO5O2; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="H4+SO5O2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5F6A21F000E9; Fri, 22 May 2026 19:43:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779479029; bh=LCMsWYweKeQIaDa+1CygcB8/DpjcoB4C7NMTyWbUNpA=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=H4+SO5O2o0zXW4iUZCvJ3XIuAbCmGf+4TTEQDoWu0ItxH6/LfzWv3oF8ksk10fdeZ kxq1SH1DqP1DvuLSN9RmgjtKdERiHGwjV5lE9joWqER3yneofwUDKzoqM3TJciLtTx m5Tf1DXm9sZyUBHvzYxrF3vB9062DY9ZWycgufd7APGJ8PcTcq5bL/MR/udaviMftj W4bYSMy0wAUs1QDJ0a0sDMhqX6ibbJfWOSlVwIpCCEtKTeiIcK/1++gdS3pz7OIrsa KJs6PUIjDX2OXSO7bhze6OFVw6YDyaDTiHeG5CNQ6LeknKqlNyOXu24+FQYaBqza05 bzbYR4pSCTDPw== Message-ID: <330fe57736b44525aeb3f9d127a1361b18038fd9.camel@kernel.org> Subject: Re: [PATCH v2 0/2] fs/pipe: reduce pipe->mutex contention by pre-allocating outside the lock From: Jeff Layton To: Breno Leitao , Alexander Viro , Christian Brauner , Jan Kara , Shuah Khan , Mateusz Guzik Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, shakeel.butt@linux.dev, oleg@redhat.com, axboe@kernel.dk, kernel-team@meta.com Date: Fri, 22 May 2026 15:43:46 -0400 In-Reply-To: <20260522-fix_pipe-v2-0-a8b35a78244e@debian.org> References: <20260522-fix_pipe-v2-0-a8b35a78244e@debian.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-05-22 at 09:44 -0700, Breno Leitao wrote: > While profiling Meta's caching code[1], I found pipe->mutex contention > on the hot path. anon_pipe_write() currently calls alloc_page() once > per page while holding pipe->mutex. The allocation can sleep doing > direct reclaim and runs memcg charging, which extends the critical > section and stalls any concurrent reader on the same mutex. >=20 > This series pre-allocates pages outside pipe->mutex in > anon_pipe_write(): for writes that span more than one full page, up > to PIPE_PREALLOC_MAX (8) pages are allocated via a per-page > alloc_page() loop before the mutex is taken. anon_pipe_get_page() > then drains the prealloc array first, falls back to the per-pipe > tmp_page[] cache, and only enters the allocator under the mutex for > the leftover pages (writes larger than PIPE_PREALLOC_MAX, single-page > writes that skip prealloc, or shortfalls when the prealloc loop > fails). Leftover prealloc pages are recycled into tmp_page[] before > unlock and any remainder is put_page()'d after unlock, keeping the > allocator out of the critical section on both sides. >=20 > alloc_pages_bulk_mempolicy() looked tempting but the bulk allocator > refuses __GFP_ACCOUNT under memcg -- it returns at most one page > when memcg_kmem_online() && (gfp & __GFP_ACCOUNT), see commit > 8dcb3060d81d ("memcg: page_alloc: skip bulk allocator for > __GFP_ACCOUNT"). A per-page loop keeps memcg accounting and the > task NUMA mempolicy honoured uniformly without open-coding the > charge. >=20 > I also vibe-coded a microbenchmark to validate the change. It sweeps > writers x readers over {1,2,5} x {1,5,10} with 64KB writes against a > 1 MB pipe and prints throughput + latency percentiles per config. >=20 > Measured on arm64 and also on x86 using virtme-ng (16 vCPUs, 64KB > writes, 1 MB pipe). The numbers below were collected on v1 > (alloc_pages_bulk()); v2's per-page loop preserves the dominant > "allocation outside the mutex" win and is expected to land in the same > range. >=20 > =3D=3D No memory pressure (10s per config) =3D=3D >=20 > Throughput in MB/s (baseline -> patched, delta): > writers readers=3D1 readers=3D5 readers= =3D10 > 1 1119 -> 1354 (+21%) 1132 -> 1195 (+6%) 1060 -> 1240 = (+17%) > 2 1162 -> 1487 (+28%) 1034 -> 1285 (+24%) 1069 -> 1213 = (+14%) > 5 1152 -> 1357 (+18%) 1021 -> 1164 (+14%) 997 -> 1239 = (+24%) >=20 > Avg write latency in ns (baseline -> patched, delta): > writers readers=3D1 readers=3D5 read= ers=3D10 > 1 55786 -> 46103 (-17%) 55164 -> 52260 (-5%) 58906 ->= 50370 (-14%) > 2 107546 -> 84011 (-22%) 120837 -> 97206 (-20%) 116860 ->= 103036 (-12%) > 5 271293 -> 230170 (-15%) 306089 -> 268429 (-12%) 313300 ->= 252232 (-19%) >=20 > Throughput improves +6% to +28% and average write latency drops 5% > to 22% across every configuration. >=20 > =3D=3D Under memory pressure (--memory-pressure, 6s per config) =3D=3D >=20 > stress-ng --vm 2 --vm-bytes 50% --vm-keep is forked alongside the > sweep so the alloc_page() calls inside anon_pipe_write() routinely > hit direct reclaim -- exactly the regime the patch targets. >=20 > Throughput in MB/s (baseline -> patched, delta): > writers readers=3D1 readers=3D5 readers=3D10 > 1 1088 -> 1438 (+32%) 996 -> 1477 (+48%) 989 -> 1194 = (+21%) > 2 1076 -> 1378 (+28%) 1007 -> 1269 (+26%) 1018 -> 1234 = (+21%) > 5 1052 -> 1311 (+25%) 986 -> 1225 (+24%) 972 -> 1249 = (+29%) >=20 > Avg write latency in ns (baseline -> patched, delta): > writers readers=3D1 readers=3D5 readers= =3D10 > 1 57397 -> 43406 (-24%) 62690 -> 42272 (-33%) 63136 ->= 52272 (-17%) > 2 116121 -> 90700 (-22%) 124098 -> 98481 (-21%) 122754 ->= 101217 (-18%) > 5 297122 -> 238322 (-20%) 316836 -> 255095 (-19%) 321496 ->= 250189 (-22%) >=20 > Throughput improves +21% to +48% and average write latency drops > 17% to 33% -- a noticeably bigger win than the no-pressure run. >=20 > That tracks: when alloc_page() has to dip into reclaim, the cost > of holding pipe->mutex across it is highest, and pulling the > allocation out of the critical section pays the most. >=20 > Link: https://www.usenix.org/system/files/conference/atc13/atc13-bronson.= pdf [1] >=20 > Signed-off-by: Breno Leitao > --- > Changes in v2: > - Switch the prealloc path from alloc_pages_bulk_mempolicy() to a > per-page alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT) loop. > - Split the prealloc work out of anon_pipe_write() into dedicated > helpers (anon_pipe_get_page_prealloc / anon_pipe_prealloc_pop / > anon_pipe_refill_tmp_pages / anon_pipe_free_pages) gathered in > struct anon_pipe_prealloc, so the write path stays readable. > - Recycle leftover prealloc pages into pipe->tmp_page[] before > unlocking > - Link to v1: https://patch.msgid.link/20260515-fix_pipe-v1-0-b14c840c755= 5@debian.org >=20 > To: Alexander Viro > To: Christian Brauner > To: Jan Kara > To: Shuah Khan > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: linux-kselftest@vger.kernel.org >=20 > --- > Breno Leitao (2): > fs/pipe: pre-allocate pages outside pipe->mutex in anon_pipe_write > selftests/pipe: add pipe_bench microbenchmark >=20 > fs/pipe.c | 105 ++++- > tools/testing/selftests/Makefile | 1 + > tools/testing/selftests/pipe/.gitignore | 1 + > tools/testing/selftests/pipe/Makefile | 9 + > tools/testing/selftests/pipe/pipe_bench.c | 616 ++++++++++++++++++++++++= ++++++ > 5 files changed, 729 insertions(+), 3 deletions(-) > --- > base-commit: e98d21c170b01ddef366f023bbfcf6b31509fa83 > change-id: 20260515-fix_pipe-c91677c187e7 >=20 > Best regards, > -- =20 > Breno Leitao Pity this can't use the bulk page allocator, but looks good otherwise. Reviewed-by: Jeff Layton