linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Message-ID: <CAHk-=wgzTgRgeAXF+6Hu2BvRzrSwPsUbUJy-vrGXGEytQGAY6Q@mail.gmail.com>
Date:   Tue, 12 Sep 2023 09:20:00 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, mgorman@...e.de,
        tglx@...utronix.de, jon.grimm@....com, bharata@....com,
        raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Mon, 11 Sept 2023 at 20:27, Matthew Wilcox <willy@...radead.org> wrote:
>
> So we do still cond_resched(), but we might go up to PMD_SIZE
> between calls.  This is new code in 6.6 so it hasn't seen use by too
> many users yet, but it's certainly bigger than the 16 pages used by
> copy_chunked_from_user().  I honestly hadn't thought about preemption
> latency.

The thing about cond_resched() is that you literally won't get anybody
who complains until the big page case is common enough that it hits
special people.

This is also a large part of why I dislike cond_resched() a lot. It's
not just that it's sprinkled randomly in our code-base, it's that it's
*found* and added so randomly.

Some developers will look at code and say "this may be a long loop"
and add it without any numbers. It's rare, but it happens.

And other than that it usually is something like the RT people who
have the latency trackers, and one particular load that they use for
testing.

Oh well. Enough kvetching. I'm not happy about it, but in the end it's
a small annoyance, not a big issue.

                Linus