linux-kernel - Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries after unmapping pages

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Date:	Wed, 10 Jun 2015 09:14:32 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	H Peter Anvin <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries
 after unmapping pages

On Wed, Jun 10, 2015 at 09:47:04AM +0200, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@...e.de> wrote:
> 
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1289,6 +1289,18 @@ enum perf_event_task_context {
> >  	perf_nr_task_contexts,
> >  };
> >  
> > +/* Track pages that require TLB flushes */
> > +struct tlbflush_unmap_batch {
> > +	/*
> > +	 * Each bit set is a CPU that potentially has a TLB entry for one of
> > +	 * the PFNs being flushed. See set_tlb_ubc_flush_pending().
> > +	 */
> > +	struct cpumask cpumask;
> > +
> > +	/* True if any bit in cpumask is set */
> > +	bool flush_required;
> > +};
> > +
> >  struct task_struct {
> >  	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
> >  	void *stack;
> > @@ -1648,6 +1660,10 @@ struct task_struct {
> >  	unsigned long numa_pages_migrated;
> >  #endif /* CONFIG_NUMA_BALANCING */
> >  
> > +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> > +	struct tlbflush_unmap_batch *tlb_ubc;
> > +#endif
> 
> Please embedd this constant size structure in task_struct directly so that the 
> whole per task allocation overhead goes away:
> 

That puts a structure (72 bytes in the config I used) within the task struct
even when it's not required. On a lightly loaded system direct reclaim
will not be active and for some processes, it'll never be active. It's
very wasteful.

-- 
Mel Gorman
SUSE Labs
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