On Wed, Jul 31, 2024 at 06:35:35PM -0300, Leonardo Bras wrote:
> On Mon, Jul 22, 2024 at 07:07:34PM +0530, neeraj.upadhyay@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> >
> > 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.
These are external functions, and the commit that uses it is just a few
farther along in the stack. Or do we now have some tool that complains
if an external function is not used anywhere?
> > [ neeraj.upadhyay: Apply Rik van Riel feedback. ]
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Imran Khan <imran.f.khan@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Leonardo Bras <leobras@xxxxxxxxxx>
> > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@xxxxxxxxxx>
> > ---
> > 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).
If it does become a problem, there are ways of taking care of it.
Just a little added complexity. ;-)
> Thanks!
And thank you for looking this over!
Thanx, Paul
