Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch

Vlastimil Babka
2021-05-28T10:28:02Z

On 5/27/21 12:52 PM, Mel Gorman wrote:
> On Wed, May 26, 2021 at 08:14:13PM +0200, Vlastimil Babka wrote:
>> > @@ -6698,11 +6717,10 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h
>> > */
>> > static void zone_set_pageset_high_and_batch(struct zone *zone)
>> > {
>> > - unsigned long new_high, new_batch;
>> > + int new_high, new_batch;
>> >
>> > - new_batch = zone_batchsize(zone);
>> > - new_high = 6 * new_batch;
>> > - new_batch = max(1UL, 1 * new_batch);
>> > + new_batch = max(1, zone_batchsize(zone));
>> > + new_high = zone_highsize(zone, new_batch);
>> >
>> > if (zone->pageset_high == new_high &&
>> > zone->pageset_batch == new_batch)
>> > @@ -8170,6 +8188,12 @@ static void __setup_per_zone_wmarks(void)
>> > zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
>> > zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>> >
>> > + /*
>> > + * The watermark size have changed so update the pcpu batch
>> > + * and high limits or the limits may be inappropriate.
>> > + */
>> > + zone_set_pageset_high_and_batch(zone);
>>
>> Hm so this puts the call in the path of various watermark related sysctl
>> handlers, but it's not protected by pcp_batch_high_lock. The zone lock won't
>> help against zone_pcp_update() from a hotplug handler. On the other hand, since
>> hotplug handlers also call __setup_per_zone_wmarks(), the zone_pcp_update()
>> calls there are now redundant and could be removed, no?
>> But later there will be a new sysctl in patch 6/6 using pcp_batch_high_lock,
>> thus that one will not be protected against the watermark related sysctl
>> handlers that reach here.
>>
>> To solve all this, seems like the static lock in setup_per_zone_wmarks() could
>> become a top-level visible lock and pcp high/batch updates could switch to that
>> one instead of own pcp_batch_high_lock. And zone_pcp_update() calls from hotplug
>> handlers could be removed.
>>
>
> Hmm, the locking has very different hold times. The static lock in
> setup_per_zone_wmarks is a spinlock that protects against parallel updates
> of watermarks and is held for a short duration. The pcp_batch_high_lock
> is a mutex that is held for a relatively long time while memory is being
> offlined and can sleep. Memory hotplug updates the watermarks without
> pcp_batch_high_lock held so overall, unifying the locking there should
> be a separate series.
>
> How about this as a fix for this patch?
>
> ---8<---
> mm/page_alloc: Disassociate the pcp->high from pcp->batch -fix
>
> Vlastimil Babka noted that __setup_per_zone_wmarks updating pcp->high
> did not protect watermark-related sysctl handlers from a parallel
> memory hotplug operations. This patch moves the PCP update to
> setup_per_zone_wmarks and updates the PCP high value while protected
> by the pcp_batch_high_lock mutex.
>
> This is a fix to the mmotm patch mm-page_alloc-disassociate-the-pcp-high-from-pcp-batch.patch.
> It'll cause a conflict with mm-page_alloc-adjust-pcp-high-after-cpu-hotplug-events.patch
> but the resolution is simply to change the caller in setup_per_zone_wmarks
> to zone_pcp_update(zone, 0)
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Looks fine. But I would also remove the redudancy introduced by this patch+fix,
as part of the patch:

online_pages()
zone_pcp_update(zone); <- this predates the patch
init_per_zone_wmark_min()
setup_per_zone_wmarks()
for_each_zone(zone)
zone_pcp_update(zone); <- new in this patch

offline_pages() similarly

In any case, for the fixed version:
Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/page_alloc.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 329b71e41db4..b1b3c66e9d88 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8199,12 +8199,6 @@ static void __setup_per_zone_wmarks(void)
> zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
> zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>
> - /*
> - * The watermark size have changed so update the pcpu batch
> - * and high limits or the limits may be inappropriate.
> - */
> - zone_set_pageset_high_and_batch(zone);
> -
> spin_unlock_irqrestore(&zone->lock, flags);
> }
>
> @@ -8221,11 +8215,19 @@ static void __setup_per_zone_wmarks(void)
> */
> void setup_per_zone_wmarks(void)
> {
> + struct zone *zone;
> static DEFINE_SPINLOCK(lock);
>
> spin_lock(&lock);
> __setup_per_zone_wmarks();
> spin_unlock(&lock);
> +
> + /*
> + * The watermark size have changed so update the pcpu batch
> + * and high limits or the limits may be inappropriate.
> + */
> + for_each_zone(zone)
> + zone_pcp_update(zone);
> }
>
> /*
>