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From:   Ankur Arora <ankur.a.arora@oracle.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        jgross@suse.com, andrew.cooper3@citrix.com,
        Joel Fernandes <joel@joelfernandes.org>,
        Youssef Esmat <youssefesmat@chromium.org>,
        Vineeth Pillai <vineethrp@google.com>,
        Suleiman Souhlal <suleiman@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Daniel Bristot de Oliveira <bristot@kernel.org>
Subject: Re: [POC][RFC][PATCH] sched: Extended Scheduler Time Slice
In-reply-to: <20231025102952.GG37471@noisy.programming.kicks-ass.net>
Date:   Wed, 25 Oct 2023 22:03:48 -0700
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Peter Zijlstra <peterz@infradead.org> writes:

> On Wed, Oct 25, 2023 at 05:42:19AM -0400, Steven Rostedt wrote:
>
>> So, bit 1 is for user space to tell the kernel "please extend me", and bit
>> two is for the kernel to tell user space "OK, I extended you, but call
>> sched_yield() when done".
>
> So what if it doesn't ? Can we kill it for not playing nice ?
>
> [ aside from it being bit 0 and bit 1 as you yourself point out, it is
>   also jarring you use a numeral for one and write out the other. ]
>
> That said, I properly hate all these things, extending a slice doesn't
> reliably work and we're always left with people demanding an ever longer
> extension.
>
> The *much* better heuristic is what the kernel uses, don't spin if the
> lock holder isn't running.

Something like vcpu_is_preempted()? That helps deal with preemption
once it has happened, but an ahead of time signal would be much more
efficient.

And, guest lock holder preemption was a big enough issue that we drop
down to test-and-set spinlocks in some cases.


Thanks

--
ankur