linux-kernel - Re: [RFC mm][PATCH 2/5] percpu cached mm counter

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Date:	Thu, 10 Dec 2009 11:51:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	minchan.kim@...il.com, mingo@...e.hu
Subject: Re: [RFC mm][PATCH 2/5] percpu cached mm counter

On Thu, 10 Dec 2009, KAMEZAWA Hiroyuki wrote:

> Now, mm's counter information is updated by atomic_long_xxx() functions if
> USE_SPLIT_PTLOCKS is defined. This causes cache-miss when page faults happens
> simultaneously in prural cpus. (Almost all process-shared objects is...)

s/prural cpus/multiple cpus simultaneously/?

> This patch implements per-cpu mm cache. This per-cpu cache is loosely
> synchronized with mm's counter. Current design is..

Some more explanation about the role of the per cpu data would be useful.

For each cpu we keep a set of counters that can be incremented using per
cpu operations. curr_mc points to the mm struct that is currently using
the per cpu counters on a specific cpu?

>   - prepare per-cpu object curr_mmc. curr_mmc containes pointer to mm and
>     array of counters.
>   - At page fault,
>      * if curr_mmc.mm != NULL, update curr_mmc.mm counter.
>      * if curr_mmc.mm == NULL, fill curr_mmc.mm = current->mm and account 1.
>   - At schedule()
>      * if curr_mm.mm != NULL, synchronize and invalidate cached information.
>      * if curr_mmc.mm == NULL, nothing to do.

Sounds like a very good idea that could be expanded and used for other
things like tracking the amount of memory used on a specific NUMA node in
the future. Through that we may get to a schedule that can schedule with
an awareness where the memory of a process is actually located.

 > By this.
>   - no atomic ops, which tends to cache-miss, under page table lock.
>   - mm->counters are synchronized when schedule() is called.
>   - No bad thing to read-side.
>
> Concern:
>   - added cost to schedule().

That is only a simple check right? Are we already touching that cacheline
in schedule? Or place that structure near other stuff touched by the
scheduer?

>
> +#if USE_SPLIT_PTLOCKS
> +
> +DEFINE_PER_CPU(struct pcp_mm_cache, curr_mmc);
> +
> +void __sync_mm_counters(struct mm_struct *mm)
> +{
> +	struct pcp_mm_cache *mmc = &per_cpu(curr_mmc, smp_processor_id());
> +	int i;
> +
> +	for (i = 0; i < NR_MM_COUNTERS; i++) {
> +		if (mmc->counters[i] != 0) {

Omit != 0?

if you change mmc->curr_mc then there is no need to set mmc->counters[0]
to zero right? add_mm_counter_fast will set the counter to 1 next?

> +static void add_mm_counter_fast(struct mm_struct *mm, int member, int val)
> +{
> +	struct mm_struct *cached = percpu_read(curr_mmc.mm);
> +
> +	if (likely(cached == mm)) { /* fast path */
> +		percpu_add(curr_mmc.counters[member], val);
> +	} else if (mm == current->mm) { /* 1st page fault in this period */
> +		percpu_write(curr_mmc.mm, mm);
> +		percpu_write(curr_mmc.counters[member], val);
> +	} else /* page fault via side-path context (get_user_pages()) */
> +		add_mm_counter(mm, member, val);

So get_user pages will not be accellerated.

> Index: mmotm-2.6.32-Dec8/kernel/sched.c
> ===================================================================
> --- mmotm-2.6.32-Dec8.orig/kernel/sched.c
> +++ mmotm-2.6.32-Dec8/kernel/sched.c
> @@ -2858,6 +2858,7 @@ context_switch(struct rq *rq, struct tas
>  	trace_sched_switch(rq, prev, next);
>  	mm = next->mm;
>  	oldmm = prev->active_mm;
> +
>  	/*
>  	 * For paravirt, this is coupled with an exit in switch_to to
>  	 * combine the page table reload and the switch backend into

Extraneous new line.

> @@ -5477,6 +5478,11 @@ need_resched_nonpreemptible:
>
>  	if (sched_feat(HRTICK))
>  		hrtick_clear(rq);
> +	/*
> +	 * sync/invaldidate per-cpu cached mm related information
> +	 * before taling rq->lock. (see include/linux/mm.h)
> +	 */
> +	sync_mm_counters_atomic();
>
>  	spin_lock_irq(&rq->lock);
>  	update_rq_clock(rq);

Could the per cpu counter stuff be placed into rq to avoid
touching another cacheline?

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