Re: [PATCH] perf/x86/amd: Warn only on new bits set

[Paul E. McKenney]

On Tue, Jun 25, 2024 at 01:57:34PM +0200, Peter Zijlstra wrote:
> On Fri, May 24, 2024 at 07:10:20AM -0700, Breno Leitao wrote:
> > Warning at every leaking bits can cause a flood of message, triggering
> > vairous stall-warning mechanisms to fire, including CSD locks, which
> > makes the machine to be unusable.
> > 
> > Track the bits that are being leaked, and only warn when a new bit is
> > set.
> > 
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
> > ---
> >  arch/x86/events/amd/core.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> > index 1fc4ce44e743..df0ba2382d13 100644
> > --- a/arch/x86/events/amd/core.c
> > +++ b/arch/x86/events/amd/core.c
> > @@ -941,11 +941,12 @@ static int amd_pmu_v2_snapshot_branch_stack(struct perf_branch_entry *entries, u
> >  static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
> >  {
> >  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> > +	static atomic64_t status_warned = ATOMIC64_INIT(0);
> > +	u64 reserved, status, mask, new_bits;
> >  	struct perf_sample_data data;
> >  	struct hw_perf_event *hwc;
> >  	struct perf_event *event;
> >  	int handled = 0, idx;
> > -	u64 reserved, status, mask;
> >  	bool pmu_enabled;
> >  
> >  	/*
> > @@ -1010,7 +1011,11 @@ static int amd_pmu_v2_handle_irq(struct pt_regs *regs)
> >  	 * the corresponding PMCs are expected to be inactive according to the
> >  	 * active_mask
> >  	 */
> > -	WARN_ON(status > 0);
> > +	if (status > 0) {
> > +		new_bits = atomic64_fetch_or(status, &status_warned) ^ atomic64_read(&status_warned);
> > +		// A new bit was set for the very first time.
> > +		WARN(new_bits, "New overflows for inactive PMCs: %llx\n", new_bits);
> > +	}
> 
> Why not just a WARN_ON_ONCE() instead? This really shouldn't be
> happening in the first place.

We did consider that, but seeing the full set of bits that shouldn't
have been happening in the first place helps with debuggging.

But is there a better way to accumulate and print the full set of
unexpected bits?

							Thanx, Paul