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 09:51:30 +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 10:21:07AM +0200, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@...e.de> wrote:
> 
> > 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.
> 
> For certain values of 'very'.
> 
>  - 72 bytes suggests that you have NR_CPUS set to 512 or so? On a kernel sized to 
>    such large systems with 1000 active tasks we are talking about about +72K of 
>    RAM...
> 

The NR_CPUS is based on the openSUSE 13.1 distro config so yes, it's large but I also
expect it to be a common configuration.

>  - Furthermore, by embedding it it gets packed better with neighboring task_struct 
>    fields, while by allocating it dynamically it's a separate cache line wasted.
> 

A separate cache line that is only used during direct reclaim when the
process is taking a large hit anyway

>  - Plus by allocating it separately you spend two cachelines on it: each slab will 
>    be at least cacheline aligned, and 72 bytes will allocate 128 bytes. So when 
>    this gets triggered you've just wasted some more RAM.
> 
>  - I mean, if it had dynamic size, or was arguably huge. But this is just a 
>    cpumask and a boolean!
> 

It gets larger with enterprise configs.

>  - The cpumask will be dynamic if you increase the NR_CPUS count any more than 
>    that - in which case embedding the structure is the right choice again.
> 

Enterprise configurations are larger. The most recent one I checked defined
NR_CPUS as 8192. If it's embedded in the structure, it means that we need
to call cpumask_clear on every fork even if it's never used. That adds
constant overhead to a fast path to avoid an allocation and a few cache
misses in a direct reclaim path. Are you certain you want that trade-off?

-- 
Mel Gorman
SUSE Labs
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