Re: [PATCH v2 2/3] locking/csd_lock: Provide an indication of ongoing CSD-lock stall

[Leonardo Bras <leobras@redhat.com>]

On Mon, Jul 22, 2024 at 07:07:34PM +0530, neeraj.upadhyay@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> If a CSD-lock stall goes on long enough, it will cause an RCU CPU
> stall warning.  This additional warning provides much additional
> console-log traffic and little additional information.  Therefore,
> provide a new csd_lock_is_stuck() function that returns true if there
> is an ongoing CSD-lock stall.  This function will be used by the RCU
> CPU stall warnings to provide a one-line indication of the stall when
> this function returns true.

I think it would be nice to also add the RCU usage here, as for the 
function being declared but not used.

> 
> [ neeraj.upadhyay: Apply Rik van Riel feedback. ]
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Imran Khan <imran.f.khan@oracle.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Leonardo Bras <leobras@redhat.com>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: Rik van Riel <riel@surriel.com>
> Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
> ---
>  include/linux/smp.h |  6 ++++++
>  kernel/smp.c        | 16 ++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/smp.h b/include/linux/smp.h
> index fcd61dfe2af3..3871bd32018f 100644
> --- a/include/linux/smp.h
> +++ b/include/linux/smp.h
> @@ -294,4 +294,10 @@ int smpcfd_prepare_cpu(unsigned int cpu);
>  int smpcfd_dead_cpu(unsigned int cpu);
>  int smpcfd_dying_cpu(unsigned int cpu);
>  
> +#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
> +bool csd_lock_is_stuck(void);
> +#else
> +static inline bool csd_lock_is_stuck(void) { return false; }
> +#endif
> +
>  #endif /* __LINUX_SMP_H */
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 81f7083a53e2..9385cc05de53 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -207,6 +207,19 @@ static int csd_lock_wait_getcpu(call_single_data_t *csd)
>  	return -1;
>  }
>  
> +static atomic_t n_csd_lock_stuck;
> +
> +/**
> + * csd_lock_is_stuck - Has a CSD-lock acquisition been stuck too long?
> + *
> + * Returns @true if a CSD-lock acquisition is stuck and has been stuck
> + * long enough for a "non-responsive CSD lock" message to be printed.
> + */
> +bool csd_lock_is_stuck(void)
> +{
> +	return !!atomic_read(&n_csd_lock_stuck);
> +}
> +
>  /*
>   * Complain if too much time spent waiting.  Note that only
>   * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
> @@ -228,6 +241,7 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
>  		cpu = csd_lock_wait_getcpu(csd);
>  		pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d, CPU#%02d released the lock.\n",
>  			 *bug_id, raw_smp_processor_id(), cpu);
> +		atomic_dec(&n_csd_lock_stuck);
>  		return true;
>  	}
>  
> @@ -251,6 +265,8 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
>  	pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %lld ns for CPU#%02d %pS(%ps).\n",
>  		 firsttime ? "Detected" : "Continued", *bug_id, raw_smp_processor_id(), (s64)ts_delta,
>  		 cpu, csd->func, csd->info);
> +	if (firsttime)
> +		atomic_inc(&n_csd_lock_stuck);
>  	/*
>  	 * If the CSD lock is still stuck after 5 minutes, it is unlikely
>  	 * to become unstuck. Use a signed comparison to avoid triggering
> -- 
> 2.40.1
> 

IIUC we have a single atomic counter for the whole system, which is 
modified in csd_lock_wait_toolong() and read in RCU stall warning.

I think it should not be an issue regarding cache bouncing because in worst 
case scenario we would have 2 modify / cpu each csd_lock_timeout (which is 
5 seconds by default).

Thanks!
Leo