Messages in this thread |  | | Date | Wed, 22 Oct 2014 23:04:31 +0200 | Subject | Re: [PATCH v2 0/12] perf/x86: implement HT leak workaround for SNB/IVB/HSW | From | Stephane Eranian <> |
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On Wed, Oct 22, 2014 at 11:12 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Oct 21, 2014 at 03:08:32PM +0200, Stephane Eranian wrote: >> On Tue, Oct 21, 2014 at 3:03 PM, Peter Zijlstra <peterz@infradead.org> wrote: >> > On Tue, Oct 21, 2014 at 02:28:06PM +0200, Stephane Eranian wrote: >> >> Peter, >> >> >> >> On Tue, Oct 21, 2014 at 1:25 PM, Peter Zijlstra <peterz@infradead.org> wrote: >> >> > >> >> > >> >> > lkml.kernel.org/r/CABPqkBRbst4sgpgE5O_VXt-CSC0VD=aP2KWA0e3Uy64tw7df3A@mail.gmail.com >> >> > >> >> > I missed that 3 lines if they were in here. >> >> > >> >> I did not put them in there because there is another problem. >> >> If you partition the generic counters 2 and 2, then some CPUs will not >> >> be able to measure some events. >> >> Unfortunately, there is no way to partition the 4 counters such that >> >> all the events can be measured by >> >> each CPU. Some events or precise sampling requires counter 2 for >> >> instance (like prec_dist). >> >> That's why I did not put this fix in. >> > >> > Ah, I wasn't thinking about a hard partition, just a limit on the number >> > of exclusive counters any one CPU can claim such as to not starve. Or is >> > that what you were talking about? I feel not being able to starve >> > another CPU is more important than a better utilization bound for >> > counter scheduling. >> >> So you're saying, just limit number of used counters to 2 regardless >> of which one they are. > > used as in marked exclusive and forced empty on the other side. > >> So sometimes, this will avoid the problem aforementioned and sometimes >> not. We can try that. > > How will this sometimes not avoid the starvation issue?
Here is a simple case: Limiting each HT to only 2 counters, can be any, 2 out of 4 possible.
HT0: you measure a MEM* in ctr2, it is started first, and it keeps running HT1: you measure PREC_DIST with PEBS (it requires ctr2)
HT0 is measuring a corrupting event on ctr2, this prevents ctr2 on HT1 from being used. HT1 is starved, it cannot measure PREC_DIST
Yes you have a quota of 2 out of 4 counters.
The quota dynamic or static can help mitigate the starvation. The only way to eliminate it is to force multiplexing even though you are using fewer counters than actually avail.
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