Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback

Leo Yan
2021-10-13T00:39:35Z

Hi German,

On Mon, Oct 11, 2021 at 04:55:37PM +0100, German Gomez wrote:
> Hi Leo,
>
> On 06/10/2021 10:51, Leo Yan wrote:
> > On Wed, Oct 06, 2021 at 10:35:20AM +0100, German Gomez wrote:
> >
> > [...]
> >
> >>> So simply say, I think the head pointer monotonically increasing is
> >>> the right thing to do in Arm SPE driver.
> >> I will talk to James about how we can proceed on this.
> > Thanks!
>
> I took this offline with James and, though it looks possible to patch
> the SPE driver to have a monotonically increasing head pointer in order
> to simplify the handling in the perf tool, it could be a breaking change
> for users of the perf_event_open syscall that currently rely on the way
> it works now.

Here I cannot create the connection between AUX head pointer and the
breakage of calling perf_event_open().

Could you elaborate what's the reason the monotonical increasing head
pointer will lead to the breakage for perf_event_open()?

> An alternative way we considered to simplify the patch is to change the
> logic inside the find_snapshot callback so that it records the entire
> contents of the aux buffer every time.
>
> What do you think?

We cannot do this way. If we send USR2 signal with very small interval,
then it's possible the hardware trace data cannot fill the full of AUX
buffer. You could use below commands for the testing and should can
observe it produces small chunk trace data:

perf record -e arm_spe_0// -S -a -- dd if=/dev/zero of=/dev/null &
PERFPID=$!
sleep 1
kill -USR2 $PERFPID
sleep .1
kill -USR2 $PERFPID

Thanks,
Leo