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 AE7E8347BC6; Fri, 22 May 2026 16:51:44 +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=1779468708; cv=none; b=u78ySRmjQgASckCVO+0bjAGrF76gBO7K0isVG0G5GCbDsEnmipTJNMt0Jwxxlqow72xLpnNk4Ffu2h9xz0CHEFwkqjXVa+mGeDXZfIJ3+eOhCPyx3g4UR5XlEclgN+eKU92wjTYzNRw6rlquv1YcHsnm8IRgp55Jc+ZMj1zZAHA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779468708; c=relaxed/simple; bh=0U2Y/R6879n4aJJI0yjDhUERksy6mXplxbT6ZhXYLKw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=qmxfEZnA/3XNG3T7/3CLogCrfBqPkjyE+1hqjwOsNqkBlXAWj5ncfkUsxIYOzUcqZTgzL6UyqeXLlc4e69u+xZKrEsl5BnbQSX+HbdLUtdZgY/X6qHTokU2moxOOywr66HK14glPJSGhLn9Hx2UCjNXaKamNf1ky6CrFgmijWLE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EP1W9ij3; 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="EP1W9ij3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 647231F000E9; Fri, 22 May 2026 16:51:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779468703; bh=3jGlCPbDWf7XBnBNv1sY+Y92Orx38IvmpNGBbUnaVpM=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=EP1W9ij3brUQaTqaHo+BD0lEWSAcFDQ1ur394RSjVHnIs511JNdrLGDTsUHnDw/0D jYI+X6NKfpHdzSuAHkDcE/l8SIse9ha1wFhaNa8gFAj7GEYX+1jXlk4Jiw5iu3/K/b RaM1vbfseXVuweE3wZqazr3omlhyoTBqskRWFbXuA6J38ZsXkWmzO/7BoVji98aOdb QHfmIYtBotVRsZ6fN40NjsdVCl5tyi6KHvqAWnnB7BXXRb9AJKVpsQP1PzkLtiyKTc BBBJyouECV6i+qbGo1gYM7jCAIFIqq8o6ZHBX4UV9M3pRp4O2F8QL6N2RCmrcfCWI2 88kJGhSwaZS1A== Message-ID: <024fd2e28a2b9bce470178b8c510ed014b9bb039.camel@kernel.org> Subject: Re: [PATCH v2 1/2] fs/pipe: pre-allocate pages outside pipe->mutex in anon_pipe_write 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 12:51:40 -0400 In-Reply-To: <20260522-fix_pipe-v2-1-a8b35a78244e@debian.org> References: <20260522-fix_pipe-v2-0-a8b35a78244e@debian.org> <20260522-fix_pipe-v2-1-a8b35a78244e@debian.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: 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: > 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. >=20 > 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. >=20 > Just pre-alloc the required pages before the lock in an array and just po= p > them inside the lock. >=20 > 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. >=20 > Signed-off-by: Breno Leitao > --- > fs/pipe.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++= ++++-- > 1 file changed, 102 insertions(+), 3 deletions(-) >=20 > diff --git a/fs/pipe.c b/fs/pipe.c > index 9841648c9cf3e..cff255217bbfe 100644 > --- a/fs/pipe.c > +++ b/fs/pipe.c > @@ -111,16 +111,76 @@ void pipe_double_lock(struct pipe_inode_info *pipe1= , > pipe_lock(pipe2); > } > =20 > -static struct page *anon_pipe_get_page(struct pipe_inode_info *pipe) > +#define PIPE_PREALLOC_MAX 8 > + > +struct anon_pipe_prealloc { > + struct page *pages[PIPE_PREALLOC_MAX]; > + unsigned int count; > +}; > + > +/* > + * Pre-allocate pages outside pipe->mutex for multi-page writes. > + * alloc_page() with GFP_HIGHUSER can sleep in reclaim and runs memcg > + * charging; doing it under the mutex stalls a concurrent reader. > + * > + * Loop alloc_page() instead of alloc_pages_bulk_*(): the bulk path refu= ses > + * __GFP_ACCOUNT under memcg (see commit 8dcb3060d81d "memcg: page_alloc= : > + * skip bulk allocator for __GFP_ACCOUNT") and silently degrades to a si= ngle > + * page. A per-page loop keeps memcg accounting and the task NUMA mempol= icy > + * honoured for every page; the per-call overhead is small compared to t= he > + * pipe->mutex hold-time being shrunk. Any shortfall is covered by the > + * in-lock alloc_page() fallback in anon_pipe_get_page(). > + */ > +static void anon_pipe_get_page_prealloc(struct anon_pipe_prealloc *preal= loc, > + size_t total_len) > +{ > + unsigned int want, i; > + struct page *page; > + > + prealloc->count =3D 0; > + if (total_len <=3D PAGE_SIZE) > + return; > + > + want =3D min_t(unsigned int, DIV_ROUND_UP(total_len, PAGE_SIZE), > + PIPE_PREALLOC_MAX); > + > + for (i =3D 0; i < want; i++) { > + page =3D alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT); > + if (!page) > + break; > + prealloc->pages[prealloc->count++] =3D page; > + } I believe alloc_pages_bulk() is supposed to be more efficient when allocating several pages at once like this. > +} > + > +static struct page *anon_pipe_prealloc_pop(struct anon_pipe_prealloc *pr= ealloc) > +{ > + if (!prealloc->count) > + return NULL; > + > + prealloc->count--; > + > + return prealloc->pages[prealloc->count]; > +} > + > +static struct page *anon_pipe_get_page(struct pipe_inode_info *pipe, > + struct anon_pipe_prealloc *prealloc) > { > + struct page *page; > + > + /* Drain prealloc first to keep tmp_page[] hot for later small writes. = */ > + page =3D anon_pipe_prealloc_pop(prealloc); > + if (page) > + return page; > + > for (int i =3D 0; i < ARRAY_SIZE(pipe->tmp_page); i++) { > if (pipe->tmp_page[i]) { > - struct page *page =3D pipe->tmp_page[i]; > + page =3D pipe->tmp_page[i]; > pipe->tmp_page[i] =3D NULL; > return page; > } > } > =20 > + /* FWIW: This is called with pipe->mutex held */ > return alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT); > } > =20 > @@ -139,6 +199,38 @@ static void anon_pipe_put_page(struct pipe_inode_inf= o *pipe, > put_page(page); > } > =20 > +/* > + * Stash leftover prealloc pages in tmp_page[] so the next write to this > + * pipe gets a hot page without entering the allocator. > + */ > +static void anon_pipe_refill_tmp_pages(struct pipe_inode_info *pipe, > + struct anon_pipe_prealloc *prealloc) > +{ > + int i, idx; > + > + if (!prealloc->count) > + return; > + > + for (i =3D 0; i < ARRAY_SIZE(pipe->tmp_page); i++) { > + if (pipe->tmp_page[i]) > + continue; > + if (!prealloc->count) > + return; > + idx =3D --prealloc->count; > + pipe->tmp_page[i] =3D prealloc->pages[idx]; > + prealloc->pages[idx] =3D NULL; > + } > +} > + > +/* Runs after mutex_unlock() to keep put_page() out of the critical sect= ion. */ > +static void anon_pipe_free_pages(struct anon_pipe_prealloc *prealloc) > +{ > + while (prealloc->count) { > + prealloc->count--; > + put_page(prealloc->pages[prealloc->count]); > + } > +} > + > static void anon_pipe_buf_release(struct pipe_inode_info *pipe, > struct pipe_buffer *buf) > { > @@ -432,6 +524,7 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *= from) > { > struct file *filp =3D iocb->ki_filp; > struct pipe_inode_info *pipe =3D filp->private_data; > + struct anon_pipe_prealloc prealloc; > unsigned int head; > ssize_t ret =3D 0; > size_t total_len =3D iov_iter_count(from); > @@ -455,6 +548,8 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *= from) > if (unlikely(total_len =3D=3D 0)) > return 0; > =20 > + anon_pipe_get_page_prealloc(&prealloc, total_len); > + > mutex_lock(&pipe->mutex); > =20 > if (!pipe->readers) { > @@ -512,7 +607,7 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *= from) > struct page *page; > int copied; > =20 > - page =3D anon_pipe_get_page(pipe); > + page =3D anon_pipe_get_page(pipe, &prealloc); > if (unlikely(!page)) { > if (!ret) > ret =3D -ENOMEM; > @@ -566,7 +661,9 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *= from) > * after waiting we need to re-check whether the pipe > * become empty while we dropped the lock. > */ > + anon_pipe_refill_tmp_pages(pipe, &prealloc); > mutex_unlock(&pipe->mutex); > + anon_pipe_free_pages(&prealloc); > if (was_empty) > wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM= ); > kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); > @@ -576,9 +673,11 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter = *from) > wake_next_writer =3D true; > } > out: > + anon_pipe_refill_tmp_pages(pipe, &prealloc); > if (pipe_is_full(pipe)) > wake_next_writer =3D false; > mutex_unlock(&pipe->mutex); > + anon_pipe_free_pages(&prealloc); > =20 > /* > * If we do do a wakeup event, we do a 'sync' wakeup, because we --=20 Jeff Layton