Re: [PATCH] smp: print only local CPU info when sched_clock goes backward

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 15, 2024 at 01:49:41PM -0400, Rik van Riel wrote:
> About 40% of all csd_lock warnings observed in our fleet appear to
> be due to sched_clock() going backward in time (usually only a little
> bit), resulting in ts0 being larger than ts2.
> 
> When the local CPU is at fault, we should print out a message reflecting
> that, rather than trying to get the remote CPU's stack trace.
> 
> Signed-off-by: Rik van Riel <riel@surriel.com>
> ---
>  kernel/smp.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f085ebcdf9e7..5656ef63ea82 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -237,6 +237,14 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
>  	if (likely(ts_delta <= csd_lock_timeout_ns || csd_lock_timeout_ns == 0))
>  		return false;
>  
> +	if (ts0 > ts2) {
> +		/* Our own sched_clock went backward; don't blame another CPU. */
> +		ts_delta = ts0 - ts2;
> +		pr_alert("sched_clock on CPU %d went backward by %llu ns\n", raw_smp_processor_id(), ts_delta);
> +		*ts1 = ts2;
> +		return false;
> +	}

So I've seen some chatter about this on IRC and was WTF, seeing this
patch I'm still WTF. What is going on with those machines?!?!