linux-kernel - Re: [tip: sched/core] sched: Fix performance regression introduced by mm

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Message-ID: <20230623081756.vVHIjkC9@linutronix.de>
Date:   Fri, 23 Jun 2023 10:17:56 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     John Johansen <john.johansen@...onical.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Aaron Lu <aaron.lu@...el.com>, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [tip: sched/core] sched: Fix performance regression introduced
 by mm_cid

On 2023-06-23 00:35:29 [-0700], John Johansen wrote:
> > 
> iirc the difference with the earlier version, is in the put case. Where in
> the earlier version, if there was lock contention the buffer would always
> get pushed onto the percpu list. With some debug patches on top we
> saw some degenerate cases where this would result in percpu lists that
> had excessive buffers on them.
> 
> So this version added a condition to force putting the buffer back
> in to the global pool if the percpu list already has 2 buffers
> cached on it.

So none of the versions perform memory allocation/ deallocation in a
preempt disabled section so it is fine from PREEMPT_RT point of view.

Sebastian