Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [RFC PATCH v2 15/20] context-tracking: Introduce work deferral infrastructure | Date | Tue, 25 Jul 2023 11:10:31 +0100 |
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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.
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