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Date:   Tue, 25 Jul 2023 15:39:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Valentin Schneider <vschneid@redhat.com>,
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Subject: Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text
 patching IPIs
Message-ID: <20230725133936.GM3765278@hirez.programming.kicks-ass.net>
References: <20230720163056.2564824-19-vschneid@redhat.com>
 <6EBAEEED-6F38-472D-BA31-9C61179EFA2F@joelfernandes.org>
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On Tue, Jul 25, 2023 at 06:49:45AM -0400, Joel Fernandes wrote:
> Interesting series Valentin. Some high-level question/comments on this one:
> 
> > On Jul 20, 2023, at 12:34 PM, Valentin Schneider <vschneid@redhat.com> wrote:
> > 
> > ﻿text_poke_bp_batch() sends IPIs to all online CPUs to synchronize
> > them vs the newly patched instruction. CPUs that are executing in userspace
> > do not need this synchronization to happen immediately, and this is
> > actually harmful interference for NOHZ_FULL CPUs.
> 
> Does the amount of harm not correspond to practical frequency of text_poke? 
> How often does instruction patching really happen? If it is very infrequent
> then I am not sure if it is that harmful.

Well, it can happen quite a bit, also from things people would not
typically 'expect' it.

For instance, the moment you create the first per-task perf event we
frob some jump-labels (and again some second after the last one goes
away).

The same for a bunch of runtime network configurations.

> > As the synchronization IPIs are sent using a blocking call, returning from
> > text_poke_bp_batch() implies all CPUs will observe the patched
> > instruction(s), and this should be preserved even if the IPI is deferred.
> > In other words, to safely defer this synchronization, any kernel
> > instruction leading to the execution of the deferred instruction
> > sync (ct_work_flush()) must *not* be mutable (patchable) at runtime.
> 
> If it is not infrequent, then are you handling the case where userland
> spends multiple seconds before entering the kernel, and all this while
> the blocking call waits? Perhaps in such situation you want the real IPI
> to be sent out instead of the deferred one?

Please re-read what Valentin wrote -- nobody is waiting on anything.