linux-kernel - [RFC PATCH 11/11] pipe: Increase the writer-wakeup threshold to reduce context-switch count [ver #3]

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Date:   Fri, 01 Nov 2019 17:35:41 +0000
From:   David Howells <dhowells@...hat.com>
To:     torvalds@...ux-foundation.org
Cc:     dhowells@...hat.com, Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        nicolas.dichtel@...nd.com, raven@...maw.net,
        Christian Brauner <christian@...uner.io>, dhowells@...hat.com,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 11/11] pipe: Increase the writer-wakeup threshold to
 reduce context-switch count [ver #3]

Increase the threshold at which the reader sends a wake event to the
writers in the queue such that the queue must be half empty before the wake
is issued rather than the wake being issued when just a single slot
available.

This reduces the number of context switches in the tests significantly,
without altering the amount of work achieved.  With my pipe-bench program,
there's a 20% reduction versus an unpatched kernel.

Suggested-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/pipe.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index aba2455caabe..9cd5cbef9552 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -324,16 +324,18 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			}
 
 			if (!buf->len) {
+				bool wake;
 				pipe_buf_release(pipe, buf);
 				spin_lock_irq(&pipe->wait.lock);
 				tail++;
 				pipe->tail = tail;
 				do_wakeup = 1;
-				if (head - (tail - 1) == pipe->max_usage)
+				wake = head - (tail - 1) == pipe->max_usage / 2;
+				if (wake)
 					wake_up_interruptible_sync_poll_locked(
 						&pipe->wait, EPOLLOUT | EPOLLWRNORM);
 				spin_unlock_irq(&pipe->wait.lock);
-				if (head - (tail - 1) == pipe->max_usage)
+				if (wake)
 					kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
 			}
 			total_len -= chars;