linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: do not do lock handoff in percpu_up

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Date: Sat, 27 Jan 2024 19:20:39 +0800
From: Hillf Danton <hdanton@...a.com>
To: Benjamin Segall <bsegall@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: do not do lock handoff in percpu_up_write

On Fri, 26 Jan 2024 12:40:43 -0800 Benjamin Segall <bsegall@...gle.com>
> 
> I'm fine with "no, fairness is more important than these performance
> numbers or mitigating already-sorta-broken situations", but it's not

Fine too because your patch is never able to escape standing ovation.

And feel free to specify the broken situations you saw.

> clear to me you've even understood the patch, because you keep only
> talking about completely different forms of starvation, and suggesting

Given woken writer in your reply and sem->ww is write waiters, there is
only one starvation in this thread.

  |> >> My patch makes the entire #4 available to writers (or new readers), so
  |> >> that the woken writer will instead get to run immediately. This is

> changes that would if anything make the situation worse.

I mitigate the starvation by making use of the known method without
any heuristic added, though, I am happy to see why it no longer works.