Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral infrastructure

[Valentin Schneider]

On 24/07/23 21:18, Frederic Weisbecker wrote:
> On Mon, Jul 24, 2023 at 05:55:44PM +0100, Valentin Schneider wrote:
>> I can make that a 'do {} while ()' instead to force at least one execution
>> of the cmpxchg().
>>
>> This is only about reducing the race window, right? If we're executing this
>> just as the target CPU is about to enter userspace, we're going to be in
>> racy territory anyway. Regardless, I'm happy to do that change.
>
> Right, it's only about narrowing down the race window. It probably don't matter
> in practice, but it's one less thing to consider for the brain :-)
>

Ack

> Also, why bothering with handling CONTEXT_IDLE?
>

I have reasons! I just swept them under the rug and didn't mention them :D
Also looking at the config dependencies again I got it wrong, but
nevertheless that means I get to ramble about it.

With NO_HZ_IDLE, we get CONTEXT_TRACKING_IDLE, so we get these
transitions:

  ct_idle_enter()
    ct_kernel_exit()
      ct_state_inc_clear_work()

  ct_idle_exit()
    ct_kernel_enter()
      ct_work_flush()

Now, if we just make CONTEXT_TRACKING_WORK depend on CONTEXT_TRACKING_IDLE
rather than CONTEXT_TRACKING_USER, we get to leverage the IPI deferral for
NO_HZ_IDLE kernels - in other words, we get to keep idle CPUs idle longer.

It's a completely different argument than reducing interference for
NOHZ_FULL userspace applications and I should have at the very least
mentioned it in the cover letter, but it's the exact same backing
mechanism.

Looking at it again, I'll probably make the CONTEXT_IDLE thing a separate
patch with a proper changelog.