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 <[email protected]> > --- > 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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