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When the PEBS interrupt threshold is larger than one record and the machine supports multiple PEBS events, the records of these events are mixed up and we need to demultiplex them. Demuxing the records is hard because the hardware is deficient. The hardware has two issues that, when combined, create impossible scenarios to demux. The first issue is that the 'status' field of the PEBS record is a copy of the GLOBAL_STATUS MSR at PEBS assist time. To see why this is a problem let us first describe the regular PEBS cycle: A) the CTRn value reaches 0: - the corresponding bit in GLOBAL_STATUS gets set - we start arming the hardware assist < some unspecified amount of time later -- this could cover multiple events of interest > B) the hardware assist is armed, any next event will trigger it C) a matching event happens: - the hardware assist triggers and generates a PEBS record this includes a copy of GLOBAL_STATUS at this moment - if we auto-reload we (re)set CTRn - we clear the relevant bit in GLOBAL_STATUS Now consider the following chain of events: A0, B0, A1, C0 The event generated for counter 0 will include a status with counter 1 set, even though its not at all related to the record. A similar thing can happen with a !PEBS event if it just happens to overflow at the right moment. The second issue is that the hardware will only emit one record for two or more counters if the event that triggers the assist is 'close'. The 'close' can be several cycles. In some cases even the complete assist, if the event is something that doesn't need retirement. For instance, consider this chain of events: A0, B0, A1, B1, C01 Where C01 is an event that triggers both hardware assists, we will generate but a single record, but again with both counters listed in the status field. This time the record pertains to both events. Note that these two cases are different but undistinguishable with the data as generated. Therefore demuxing records with multiple PEBS bits (we can safely ignore status bits for !PEBS counters) is impossible. Furthermore we cannot emit the record to both events because that might cause a data leak -- the events might not have the same privileges -- so what this patch does is discard such events. The assumption/hope is that such discards will be rare. Here lists some possible ways you may get high discard rate. - when you count the same thing multiple times. But it is not a useful configuration. - you can be unfortunate if you measure with a userspace only PEBS event along with either a kernel or unrestricted PEBS event. Imagine the event triggering and setting the overflow flag right before entering the kernel. Then all kernel side events will end up with multiple bits set. Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com> Signed-off-by: Kan Liang <kan.liang@intel.com> [ Changelog improvements. ] Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: acme@infradead.org Cc: eranian@google.com Link: http://lkml.kernel.org/r/1430940834-8964-4-git-send-email-kan.liang@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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