Messages in this thread | | | Date | Tue, 19 Jun 2018 14:00:44 +0300 | From | Alexander Shishkin <> | Subject | Re: [PATCH v1 4/6] perf: Allow using AUX data in perf samples |
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On Thu, Jun 14, 2018 at 09:47:20PM +0200, Peter Zijlstra wrote: > On Tue, Jun 12, 2018 at 10:51:15AM +0300, Alexander Shishkin wrote: > > @@ -6112,6 +6219,32 @@ void perf_prepare_sample(struct perf_event_header *header, > > > > if (sample_type & PERF_SAMPLE_PHYS_ADDR) > > data->phys_addr = perf_virt_to_phys(data->addr); > > + > > + if (sample_type & PERF_SAMPLE_AUX) { > > + u64 size; > > + > > + header->size += sizeof(u64); /* size */ > > + > > + /* > > + * Given the 16bit nature of header::size, an AUX sample can > > + * easily overflow it, what with all the preceding sample bits. > > + * Make sure this doesn't happen by using up to U16_MAX bytes > > + * per sample in total (rounded down to 8 byte boundary). > > + */ > > + size = min_t(size_t, U16_MAX - header->size, > > + event->attr.aux_sample_size); > > + size = rounddown(size, 8); > > + size = perf_aux_sample_size(event, data, size); > > + > > + WARN_ON_ONCE(size + header->size > U16_MAX); > > + header->size += size; > > + } > > + /* > > + * If you're adding more sample types here, you likely need to do > > + * something about the overflowing header::size, like repurpose the > > + * lowest 3 bits of size, which should be always zero at the moment. > > + */ > > Bugger yes.. I fairly quickly (but still too late) realized we should've > used that u16 in u64 increments to allow up to 512K instead of 64K > events. > > Still, even 64K samples are pretty terrifyingly huge. They'll be > _sloowww_. > > In any case, I suppose we can grab one of the attribute bits to rev. the > output format -- a la sample_id_all. Do we really want to do that? 512K > samples.... *shudder*.
Well, as far as PT goes, even a 4K sample carries a thousands of branches, I can't imagine ever needing more than 64K, but I didn't really want to make the call. Same goes for the width of aux_sample_size, which should match what we can theoretically output.
Also as a thought, we could use the perf_adjust_period() to also reduce the sample size dynamically if it takes too long to copy.
Regards, -- Alex
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