linux-kernel - Re: [PATCH 1/2] pipe: change pipe_write() to never add a zero-sized buffer

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Message-ID: <b050f92e-4117-4e93-8ec6-ec595fd8570a@amd.com>
Date: Mon, 10 Feb 2025 05:09:12 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Oleg Nesterov <oleg@...hat.com>
CC: Christian Brauner <brauner@...nel.org>, Jeff Layton <jlayton@...nel.org>,
	David Howells <dhowells@...hat.com>, "Gautham R. Shenoy"
	<gautham.shenoy@....com>, Mateusz Guzik <mjguzik@...il.com>, Neeraj Upadhyay
	<Neeraj.Upadhyay@....com>, Oliver Sang <oliver.sang@...el.com>, "Swapnil
 Sapkal" <swapnil.sapkal@....com>, WangYuli <wangyuli@...ontech.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Linus
 Torvalds" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] pipe: change pipe_write() to never add a zero-sized
 buffer

Hello Oleg,

On 2/10/2025 12:45 AM, Oleg Nesterov wrote:
> On 02/09, Linus Torvalds wrote:
>>
>> On Sun, 9 Feb 2025 at 10:45, Oleg Nesterov <oleg@...hat.com> wrote:
>>>
>>> Again, lets look eat_empty_buffer().
>>>
>>> The comment says "maybe it's empty" but how/why can this happen ?
>>
>> WHY DO YOU CARE?
> 
> Because it looks unclear/confusing, and I think it can confuse other
> readers of this code. Especially after 1/2.
> 
>> So here's the deal: either you
> ...
>>   (b) you DON'T convince yourself that that is true, and you leave
>> eat_empty_buffer() alone.
> 
> Yes, I failed to convince myself that fs/splice.c can never add an
> empty bufer. Although it seems to me it should not.
> 
>> In contrast, the "eat_empty_buffer()" case just saying "if it's an
>> empty buffer, it doesn't satisfy my needs, so I'll just release the
>> empty buffer and go on".
> 
> ... without wakeup_pipe_writers().
> 
> OK, nevermind, I see your point even if I do not 100% agree.
> 
> I'll send v2 without WARN_ON() and without 2/2.

Went ahead and tested that version on top of mainline with your
previous change to skip updating {a,c,m}time, here are the results:

==================================================================
Test          : sched-messaging
Units         : Normalized time in seconds
Interpretation: Lower is better
Statistic     : AMean
==================================================================
Case:       mainline + no-acm_time[pct imp](CV)   patched[pct imp](CV)
  1-groups     1.00 [ -0.00]( 7.19)                0.95 [  4.90](12.39)
  2-groups     1.00 [ -0.00]( 3.54)                1.02 [ -1.92]( 6.55)
  4-groups     1.00 [ -0.00]( 2.78)                1.01 [ -0.85]( 2.18)
  8-groups     1.00 [ -0.00]( 1.04)                0.99 [  0.63]( 0.77)
16-groups     1.00 [ -0.00]( 1.02)                1.00 [ -0.26]( 0.98)

I don't see any regression / improvements from a performance standpoint
on my 3rd Generation EPYC system (2 x 64C/128T. boost on, C2 disabled)
Feel free to include:

Tested-by: K Prateek Nayak <kprateek.nayak@....com>

> 
> Oleg.
> 

-- 
Thanks and Regards,
Prateek