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

From: Peter Zijlstra
Date: Mon Sep 08 2014 - 03:17:25 EST


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.

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