Hi Leo,
在 2023/8/14 15:16, Leo Yan 写道:
On Fri, Aug 11, 2023 at 07:05:20PM +0100, Marc Zyngier wrote:
Huang Shijie reports that, when profiling a guest from the host
with a number of events that exceeds the number of available
counters, the reported counts are wildly inaccurate. Without
the counter oversubscription, the reported counts are correct.
Their investigation indicates that upon counter rotation (which
takes place on the back of a timer interrupt), we fail to
re-apply the guest EL0 enabling, leading to the counting of host
events instead of guest events.
Seems to me, it's not clear for why the counter rotation will cause
the issue.
In the example shared by Shijie in [1], the cycle counter is enabled for
both host and guest, and cycle counter is a dedicated event which does
not share counter with other events. Even there have counter rotation,
it should not impact the cycle counter.
Just take a simple case:
perf stat -e cycles:G,cycles:H, e2,e3,e4,e5,e6,e7 ....
Assume we have 8 events, but PMU only privides 7 counters(cycle + 6 normal)
1.) The initial:
event 0 (cycles:G) ---> used cycle counter
event 1 (cycles:H) ---> used counter 1
event 2 ---> used counter 2
event 3 ---> used counter 3
event 4 ---> used counter 4
event 5 ---> used counter 5
event 6---> used counter 6
2.) After the event rotation , the event0 will put to the tail of the
list, please see rotate_ctx()
the first round, it will like this:
event 1(cycles:H) ---> used cycle counter
event 2 ---> used counter 1
event 3 ---> used counter 2
event 4 ---> used counter 3
event 5 ---> used counter 4
event 6 ---> used counter 5
event 7 ---> used counter 6
3.) Rotation it again, event 1 will in the tail.
In the second round, it will like this:
evnet 0(cycles:G) ---> used cycle counter
event 2 ---> used counter 1
event 3 ---> used counter 2
event 4 ---> used counter 3
event 5 ---> used counter 4
event 6 ---> used counter 5
event 7 ---> used counter 6
4.) Rotation it again, in the third round, it will like this:
evnet 0(cycles:G) ---> used cycle counter
event 3 ---> used counter 1
event 4 ---> used counter 2
event 5 ---> used counter 3
event 6 ---> used counter 4
event 7 ---> used counter 5
event 2(cycles:H) ---> used counter 6
....
So it will impact the cycle counter..:)
Thanks
Huang Shijie
