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 09:47:04 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
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


* 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:

> +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +/*
> + * Allocate the control structure for batch TLB flushing. An allocation
> + * failure is harmless as the reclaimer will send IPIs where necessary.
> + * A GFP_KERNEL allocation from this context is normally not advised but
> + * we are depending on PF_MEMALLOC (set by direct reclaim or kswapd) to
> + * limit the depth of the call.
> + */
> +static void alloc_tlb_ubc(void)
> +{
> +	if (!current->tlb_ubc)
> +		current->tlb_ubc = kzalloc(sizeof(struct tlbflush_unmap_batch),
> +						GFP_KERNEL | __GFP_NOWARN);
> +}
> +#else
> +static inline void alloc_tlb_ubc(void)
> +{
> +}
> +#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> +
>  /*
>   * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
>   */
> @@ -2152,6 +2174,8 @@ static void shrink_lruvec(struct lruvec *lruvec, int swappiness,
>  	scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
>  			 sc->priority == DEF_PRIORITY);
>  
> +	alloc_tlb_ubc();
> +
>  	blk_start_plug(&plug);
>  	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
>  					nr[LRU_INACTIVE_FILE]) {

the whole patch series will become even simpler.

Thanks,

	Ingo
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