linux-kernel - Re: sched: softlockups in multi_cpu

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Date:	Fri, 06 Mar 2015 11:55:02 -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 11:32 -0800, Linus Torvalds wrote:

> IOW, I wonder if we could special-case the common non-IO
> fault-handling path something along the lines of:
> 
>  - look up the vma in the vma lookup cache

But you'd still need mmap_sem there to at least get the VMA's first
value.

>  - look up the page in the page cache
>  - get the page table spinlock
>  - re-check the vma now (it ends up being stable if it can't be torn
> down due to the page table spinlock)
> 
> because I suspect that page faults are the biggest users of that
> mmap_sem, and we could probably handle a fairly large common case
> (making it simpler by special-casing it and punting in any even
> _slightly_ complicated situations) without even getting the semaphore
> at all, since we have to serialize on the actual page table *anyway*.
> 
> Basically, to me, the whole "if a lock is so contended that we need to
> play locking games, then we should look at why we *use* the lock,
> rather than at the lock itself" is a religion.
> 
>                          Linus
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