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SubjectRe: [RFC 1/5] mm, page_alloc: clean up pageset high and batch update
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On 9/10/20 10:31 AM, Oscar Salvador wrote:
> On Mon, Sep 07, 2020 at 06:36:24PM +0200, Vlastimil Babka wrote:
>
>> -/*
>> - * pageset_set_high() sets the high water mark for hot per_cpu_pagelist
>> - * to the value high for the pageset p.
>> - */
>> -static void pageset_set_high(struct per_cpu_pageset *p,
>> - unsigned long high)
>> -{
>> - unsigned long batch = max(1UL, high / 4);
>> - if ((high / 4) > (PAGE_SHIFT * 8))
>> - batch = PAGE_SHIFT * 8;
>> -
>> - pageset_update(&p->pcp, high, batch);
>> + pageset_update(&p->pcp, 0, 1);
>> }
>
> Could we restore the comment we had in pageset_set_high, and maybe
> update it to match this new function? I think it would be useful.

Same as David, I didn't find the comment useful at all. But I can try writing a
better one instead.

>>
>> static void pageset_set_high_and_batch(struct zone *zone,
>> - struct per_cpu_pageset *pcp)
>> + struct per_cpu_pageset *p)
>> {
>> - if (percpu_pagelist_fraction)
>> - pageset_set_high(pcp,
>> - (zone_managed_pages(zone) /
>> - percpu_pagelist_fraction));
>> - else
>> - pageset_set_batch(pcp, zone_batchsize(zone));
>> + unsigned long new_high;
>> + unsigned long new_batch;
>> + int fraction = READ_ONCE(percpu_pagelist_fraction);
>
> Why the READ_ONCE? In case there is a parallel update so things to get
> messed up?

Yeah, I think online of a new zone can race with sysctl update to
percpu_pagelist_fraction, because online doesn't take pcp_batch_high_lock...
until patch 5. But you're right this should be separate.

> as I said, I'd appreciate a comment in pageset_set_high_and_batch to be
> restored and updated, otherwise:
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Thanks!

> Thanks
>

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