linux-kernel - Re: [patch V2 3/7] futex: Add op for hash preallocation

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Date:	Sat, 7 May 2016 10:47:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhart@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation

On Fri, 6 May 2016, Darren Hart wrote:
> On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> > From: Sebastian Siewior <bigeasy@...utronix.de>
> > 
> > The per process hash is allocated on the fly at the first futex operation of a
> > process. The size of the hash is determined by a system wide default setting
> > controlled by the sys admin, This is suboptimal for RT applications and
> > applications with pathological futex abuse,
> > 
> >  - For RT applications its important to allocate the per process hash before the
> >    first futex operation to avoid the allocation on the first futex operation.
> > 
> >  - For pathological applications which use gazillions of futexes its useful to
> >    allocate a hash greater than the default hash size.
> 
> "it's" or preferably "it is"
> 
> > 
> > Add a futex op which allows to preallocate the hash with the requested
> 
> "allows for preallocating"
> 
> > size. The size is limited by the systemwide maximum hash size, which can be
> 
> system-wide
> 
> > set by the admin. The requested size is rounded up to the next order of 2.
> > 
> > The function can be called several times, but ony the first call results in a
> > hash allocation of the requested size as there is no non-intrusive way to
> > reallocate/rehash in a multithreaded application.
> > 
> > Note, that this call must be issued before the first futex operation in the
> > process because that would automatically allocate the default sized hash.
> 
> So this seems like it could be tricky for the user as system libraries, like
> glibc, make use of futexes. Can we guarantee that "sys_futex" is not called by
> the time main() is called?

To the extent of my testing I never observed that the hash was automatically
created when I called futex(PREALLOC) right away in main. But yes, that might
need some thought.

Thanks,

	tglx