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

[Peter Zijlstra <peterz@infradead.org>]

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@kernel.org>
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  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.