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

Peter Zijlstra
2023-07-25T13:39:45Z

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.