Re: [PATCH v2 03/10] mm/page_alloc: insert memory barriers to allow async update of pcp batch and high

[Gilad Ben-Yossef <gilad@benyossef.com>]

On Wed, Apr 10, 2013 at 9:19 AM, Gilad Ben-Yossef <gilad@benyossef.com> wrote:
> On Wed, Apr 10, 2013 at 2:28 AM, Cody P Schafer <cody@linux.vnet.ibm.com> wrote:
>> In pageset_set_batch() and setup_pagelist_highmark(), ensure that batch
>> is always set to a safe value (1) prior to updating high, and ensure
>> that high is fully updated before setting the real value of batch.
>>
>> Suggested by Gilad Ben-Yossef <gilad@benyossef.com> in this thread:
>>
>>         https://lkml.org/lkml/2013/4/9/23
>>
>> Also reproduces his proposed comment.
>>
>> Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
>> ---
>>  mm/page_alloc.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d259599..a07bd4c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4007,11 +4007,26 @@ static int __meminit zone_batchsize(struct zone *zone)
>>  #endif
>>  }
>>
>> +static void pageset_update_prep(struct per_cpu_pages *pcp)
>> +{
>> +       /*
>> +        * We're about to mess with PCP in an non atomic fashion.  Put an
>> +        * intermediate safe value of batch and make sure it is visible before
>> +        * any other change
>> +        */
>> +       pcp->batch = 1;
>> +       smp_wmb();
>> +}
>> +
>>  /* a companion to setup_pagelist_highmark() */
>>  static void pageset_set_batch(struct per_cpu_pageset *p, unsigned long batch)
>>  {
>>         struct per_cpu_pages *pcp = &p->pcp;
>> +       pageset_update_prep(pcp);
>> +
>>         pcp->high = 6 * batch;
>> +       smp_wmb();
>> +
>>         pcp->batch = max(1UL, 1 * batch);
>>  }
>>
>> @@ -4039,7 +4054,11 @@ static void setup_pagelist_highmark(struct per_cpu_pageset *p,
>>         struct per_cpu_pages *pcp;
>>
>>         pcp = &p->pcp;
>> +       pageset_update_prep(pcp);
>> +
>>         pcp->high = high;
>> +       smp_wmb();
>> +
>>         pcp->batch = max(1UL, high/4);
>>         if ((high/4) > (PAGE_SHIFT * 8))
>>                 pcp->batch = PAGE_SHIFT * 8;
>> --
>> 1.8.2
>>
>
> That is very good.
> However, now we've created a "protocol" for updating ->high and ->batch:
>
> 1. Call pageset_update_prep(pcp)
> 2. Update ->high
> 3. smp_wmb()
> 4. Update ->batch
>
> But that protocol is not documented anywhere and someone  that reads
> the code two
> years from now will not be aware of it or why it is done like that.
>
> How about if we create:
>
> /*
> * pcp->high and pcp->batch values are related and dependent on one another:
> * ->batch must never be higher then ->high.
> * The following function updates them in a safe manner without a
> costly atomic transaction.
> */
> static void pageset_update(struct per_cpu_pages *pcp, unsigned int
> high, unsigned int batch)
> {
>        /* start with a fail safe value for batch */
>        pcp->batch = 1;
>        smp_wmb();
>
>        /* Update high, then batch, in order */
>        pcp->high = high;
>        smp_wmb();
>        pcp->batch = batch;
> }
>
> And use that at the update sites? then the protocol becomes explicit.

Oh, and other then that it looks good to me, so assuming you do that -

Reviewed-By: Gilad Ben-Yossef <gilad@benyossef.com>

Many thanks,
Gilad


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Gilad Ben-Yossef
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