linux-kernel - Re: [PATCH] smp: add a best_effort version of smp_call_function

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Message-ID: <20210419191712.GB26214@worktop.programming.kicks-ass.net>
Date:   Mon, 19 Apr 2021 21:17:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Luigi Rizzo <lrizzo@...gle.com>
Cc:     linux-kernel@...r.kernel.org, axboe@...nel.dk, paulmck@...nel.org
Subject: Re: [PATCH] smp: add a best_effort version of
 smp_call_function_many()

On Mon, Apr 19, 2021 at 11:44:55AM -0700, Luigi Rizzo wrote:
> Regardless of the 'wait' argument, smp_call_function_many() must spin
> if any of the target CPUs have their csd busy waiting to be processed
> for a previous call. This may cause high tail latencies e.g. when some
> of the target CPUs are running functions that disable interrupts for a
> long time; getrusage() is one possible culprit.
> 
> Here we introduce a variant, __smp_call_function_many(), that adds
> a third 'best_effort' mode to the two existing ones (nowait, wait).
> In best effort mode, the call will skip CPUs whose csd is busy, and if
> any CPU is skipped it returns -EBUSY and the set of busy in the mask.
> This allows the caller to decide how to proceed, e.g. it might retry at
> a later time, or use a private csd, etc..
> 
> The new function is a compromise to avoid touching existing callers of
> smp_call_function_many(). If the feature is considered interesting, we
> could even replace the 'wait' argument with a ternary 'mode' in all
> smp_call_function_*() and derived methods.

I don't see a user of this...