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Message-ID: <1e254a35-d0c2-8d41-f020-21694945911a@joelfernandes.org>
Date:   Tue, 25 Jul 2023 13:41:32 -0400
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Subject: Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text
 patching IPIs
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To:     Valentin Schneider <vschneid@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org,
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        =?UTF-8?Q?Thomas_Wei=c3=9fschuh?= <linux@weissschuh.net>,
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References: <20230720163056.2564824-19-vschneid@redhat.com>
 <6EBAEEED-6F38-472D-BA31-9C61179EFA2F@joelfernandes.org>
 <xhsmhtttsru2s.mognet@vschneid.remote.csb>
From:   Joel Fernandes <joel@joelfernandes.org>
In-Reply-To: <xhsmhtttsru2s.mognet@vschneid.remote.csb>
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On 7/25/23 09:36, Valentin Schneider wrote:
> On 25/07/23 06:49, 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.
>>
> 
> Being pushed over a latency threshold *once* is enough to impact the
> latency evaluation of your given system/application.
> 
> It's mainly about shielding the isolated, NOHZ_FULL CPUs from whatever the
> housekeeping CPUs may be up to (flipping static keys, loading kprobes,
> using ftrace...) - frequency of the interference isn't such a big part of
> the reasoning.

Makes sense.

>>> 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?
>>
> 
> The blocking call only waits for CPUs for which it queued a CSD. Deferred
> calls do not queue a CSD thus do not impact the waiting at all. See
> smp_call_function_many_cond().

Ah I see you are using on_each_cpu_cond(). I should have gone through
the other patch before making noise.

thanks,

 - Joel