linux-kernel - Re: sched: softlockups in multi_cpu

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Date:	Fri, 06 Mar 2015 11:20:43 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>
Subject: Re: sched: softlockups in multi_cpu_stop

On Fri, 2015-03-06 at 11:05 -0800, Linus Torvalds wrote:
> On Fri, Mar 6, 2015 at 10:57 AM, Jason Low <jason.low2@...com> wrote:
> >
> > Right, the can_spin_on_owner() was originally added to the mutex
> > spinning code for optimization purposes, particularly so that we can
> > avoid adding the spinner to the OSQ only to find that it doesn't need to
> > spin. This function needing to return a correct value should really only
> > affect performance, so yes, lockups due to this seems surprising.
> 
> Well, softlockups aren't about "correct behavior". They are about
> certain things not happening in a timely manner.
> 
> Clearly the mutex code now tries to hold on to the CPU too aggressively.

This patch was a performance "fix" for rwsems, where it works well
mutexes.

> 
> At some point people need to admit that busy-looping isn't always a
> good idea. Especially if
> 
>  (a) we could idle the core instead
> 
>  (b) the tuning has been done based on som especial-purpose benchmark
> that is likely not realistic
> 
>  (c) we get reports from people that it causes problems.
> 
> In other words: Let's just undo that excessive busy-looping. The
> performance numbers were dubious to begin with. Real scalability comes
> from fixing the locking, not from trying to play games with the locks
> themselves. Particularly games that then cause problems.

I obviously agree with all those points, however fyi most of the testing
on rwsems I do includes scaling address space ops stressing the
mmap_sem, which is a real world concern. So while it does include
microbenchmarks, it is not guided by them.

Thanks,
Davidlohr

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