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SubjectRe: [GIT PULL] perf x86 updates for v3.20
On 02/15/2015 11:48 PM, Ingo Molnar wrote:
> Linus,
>
> Please pull the latest perf-core-for-linus git tree from:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git perf-core-for-linus
>
> # HEAD: a66734297f78707ce39d756b656bfae861d53f62 perf/x86: Add /sys/devices/cpu/rdpmc=2 to allow rdpmc for all tasks

[...]

> The extra CR4 manipulation adds ~ <50ns to the context
> switch cost between rdpmc-capable and rdpmc-non-capable
> mms.

That's about the best I could benchmark, too -- if it was more than
about 50ns, I'm pretty sure I wouldn't seen a difference, but, as it
stands, it seems to have been lost in the noise. Maybe I should find a
better benchmark.

In any event, this series is probably a mixed bag performance-wise. In
the best base, there's a small extra cost in context switches, and, when
switching PCE, there's a CR4 write. On SVM guests, the CR4 write will suck.

To balance that out, I removed a CR4 read from VMX entry and from global
TLB flushes. The former mostly fixes a performance regression from a
security fix a few releases back, and the I expect that the latter will
more than offset the added context switch overhead (especially on SVM
guests, where even CR4 reads exit AFAIK).

Anyway, I tried and failed to detect any difference at all. Context
switch timing was very noisy for me.

--Andy


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