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Context switches with perf LBR call stack context are fairly expensive because they do a lot of MSR writes. Currently we unconditionally do the expensive operation when LBR call stack is enabled. It's not necessary for some common cases, e.g task -> other kernel thread -> same task. The LBR registers are not changed, hence they don't need to be rewritten/restored. Introduce per-CPU variables to track the last LBR call stack context. If the same context is scheduled in, the rewrite/restore is not required, with the following two exceptions: - The LBR registers may be modified by a normal LBR event, i.e., adding a new LBR event or scheduling an existing LBR event. In both cases, the LBR registers are reset first. The last LBR call stack information is cleared in intel_pmu_lbr_reset(). Restoring the LBR registers is required. - The LBR registers are initialized to zero in C6. If the LBR registers which TOS points is cleared, C6 must be entered while swapped out. Restoring the LBR registers is required as well. These exceptions are not common. Signed-off-by: Kan Liang <kan.liang@linux.intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Stephane Eranian <eranian@google.com> Cc: Vince Weaver <vincent.weaver@maine.edu> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: acme@kernel.org Cc: eranian@google.com Link: https://lore.kernel.org/lkml/1528213126-4312-2-git-send-email-kan.liang@linux.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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