Re: [RFC PATCH v2 18/20] context_tracking,x86: Defer kernel text patching IPIs

Joel Fernandes
2023-07-25T17:41:40Z

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