Re: [PATCH v4 04/22] perf: Add a capability for AUX_NO_SG pmus to do software double buffering

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Wed, Aug 20, 2014 at 03:36:01PM +0300, Alexander Shishkin wrote:
>> For pmus that don't support scatter-gather for AUX data in hardware, it
>> might still make sense to implement software double buffering to avoid
>> losing data while the user is reading data out. For this purpose, add
>> a pmu capability that guarantees multiple high-order chunks for AUX buffer,
>> so that the pmu driver can do switchover tricks.
>
> Please expand this with more detail on how to use this.

Sure.

>> Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
>> ---
>>  include/linux/perf_event.h  |  1 +
>>  kernel/events/ring_buffer.c | 15 ++++++++++++++-
>>  2 files changed, 15 insertions(+), 1 deletion(-)
>> 
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index fe10bf6f94..1e7b659b49 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -172,6 +172,7 @@ struct perf_event;
>>   */
>>  #define PERF_PMU_CAP_NO_INTERRUPT		0x01
>>  #define PERF_PMU_CAP_AUX_NO_SG			0x02
>> +#define PERF_PMU_CAP_AUX_SW_DOUBLEBUF		0x04
>>  
>>  /**
>>   * struct pmu - generic performance monitoring unit
>> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
>> index d10919ca42..f5ee3669f8 100644
>> --- a/kernel/events/ring_buffer.c
>> +++ b/kernel/events/ring_buffer.c
>> @@ -286,9 +286,22 @@ int rb_alloc_aux(struct ring_buffer *rb, struct perf_event *event,
>>  	if (!has_aux(event))
>>  		return -ENOTSUPP;
>>  
>> -	if (event->pmu->capabilities & PERF_PMU_CAP_AUX_NO_SG)
>> +	if (event->pmu->capabilities & PERF_PMU_CAP_AUX_NO_SG) {
>>  		order = get_order(nr_pages * PAGE_SIZE);
>>  
>> +		/*
>> +		 * PMU requests more than one contiguous chunks of memory
>> +		 * for SW double buffering
>> +		 */
>> +		if ((event->pmu->capabilities & PERF_PMU_CAP_AUX_SW_DOUBLEBUF) &&
>> +		    !overwrite) {
>> +			if (!order)
>> +				return -EINVAL;
>> +
>> +			order--;
>> +		}
>> +	}
>
> In particular this looks like it will allocate double the total amount
> of pages and 'loose' half of them. There is no corresponding code in the
> free path to collect them.

This code makes the biggest high order allocation no bigger than half of
the total requested size. Then, when I allocate the high-order chunks, I
do a split_page() on them and everywhere else in the code they are
treated as individual pages, including the free path. So this patch has
no implication on freeing. Is this your concern?

Regards,
--
Alex
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