On Wed, Mar 24, 2021 at 11:15 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 24 Mar 2021 15:04:16 +0200
> "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@xxxxxxxxx> wrote:
>
> > In host-guest tracing session, all peers should use the same tracing
> > clock. If there is no user configured trace clock, the current logic
> > assumes "local" clock for the session. This could be wrong, as other
> > clock than "local" could be already configured on the host, before
> > running trace-cmd. The default clock for host-guest tracing session
> > should be rertieved from the host's "trace_clock" file.
>
> Actually this is wrong. If clock is not specified, you should check the
> first instance, because if the user does:
>
> # trace-cmd start -B foo -p nop -C myclock
>
> # trace-cmd record -B foo -e kvm -e sched -A Guest -e all
>
> It should not use the clock from the top instance, but instead the first
> instance.
Yes, the implementation uses the clock from the first non-guest
instance, the description of the patch is not correct.
>
> I'm trying to make sure tha trace-cmd does not affect or even use the top
> instance if it is not part of the command line.
>
> >
> > Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@xxxxxxxxx>
> > ---
> > tracecmd/trace-record.c | 40 ++++++++++++++++++++++++++++++----------
> > 1 file changed, 30 insertions(+), 10 deletions(-)
> >
> > diff --git a/tracecmd/trace-record.c b/tracecmd/trace-record.c
> > index f90fdbe4..2fc6723a 100644
> > --- a/tracecmd/trace-record.c
> > +++ b/tracecmd/trace-record.c
> > @@ -6455,11 +6455,12 @@ static void get_tsc_offset(struct common_record_context *ctx)
> >
> > static void set_tsync_params(struct common_record_context *ctx)
> > {
> > - const char *clock = ctx->clock;
> > struct buffer_instance *instance;
> > int shift, mult;
> > bool force_tsc = false;
> > + char *clock = NULL;
> >
> > + if (!ctx->clock) {
> > /*
> > * If no clock is configured &&
> > * KVM time sync protocol is available &&
> > @@ -6468,18 +6469,35 @@ static void set_tsync_params(struct common_record_context *ctx)
> > * force using the x86-tsc clock for this host-guest tracing session
> > * and store TSC to nsec multiplier and shift.
> > */
> > - if (!clock && tsync_proto_is_supported("kvm") &&
> > - clock_is_supported(NULL, TSC_CLOCK) &&
> > - !get_tsc_nsec(&shift, &mult) && mult) {
> > - clock = TSC_CLOCK;
> > - ctx->tsc2nsec.mult = mult;
> > - ctx->tsc2nsec.shift = shift;
> > - ctx->tsc2nsec.offset = get_clock_now(TSC_CLOCK);
> > - force_tsc = true;
> > + if (tsync_proto_is_supported("kvm") &&
> > + clock_is_supported(NULL, TSC_CLOCK) &&
>
> So we want to test the first instance, not the top one.
I assume that if the top instance supports "x86-tsc" clock, then any
instance should support it also ? Is it possible that supported clocks
can be different in each instance ?
>
> -- Steve
>
> > + !get_tsc_nsec(&shift, &mult) && mult) {
> > + clock = strdup(TSC_CLOCK);
> > + if (!clock)
> > + die("Cannot not allocate clock");
> > + ctx->tsc2nsec.mult = mult;
> > + ctx->tsc2nsec.shift = shift;
> > + ctx->tsc2nsec.offset = get_clock_now(TSC_CLOCK);
> > + force_tsc = true;
> > + } else {
> > + /*
> > + * Else, use the current clock of the first host instance
> > + */
> > + for_all_instances(instance) {
> > + if (is_guest(instance))
> > + continue;
> > + clock = tracefs_get_clock(instance->tracefs);
> > + break;
This is the loop that gets the clock from the first non-guest
instance, in case there is no "-C ..." option in the command line.
> > + }
> > + }
> > + } else {
> > + clock = strdup(ctx->clock);
> > + if (!clock)
> > + die("Cannot not allocate clock");
> > }
> >
> > if (!clock && !ctx->tsync_loop_interval)
> > - return;
> > + goto out;
> > for_all_instances(instance) {
> > if (clock && !(instance->flags & BUFFER_FL_HAS_CLOCK)) {
> > /* use the same clock in all tracing peers */
> > @@ -6501,6 +6519,8 @@ static void set_tsync_params(struct common_record_context *ctx)
> > }
> > instance->tsync_loop_interval = ctx->tsync_loop_interval;
> > }
> > +out:
> > + free(clock);
> > }
> >
> > /*
>
--
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center
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