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

[Peter Zijlstra]

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.

> 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.

Attachment: pgplFfRzppT3W.pgp
Description: PGP signature