linux-kernel - Re: [PATCH rcu 04/11] kernel/smp: Provide boot-time timeout for CSD lock diagnostics

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Date:   Tue, 19 Apr 2022 09:44:58 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH rcu 04/11] kernel/smp: Provide boot-time timeout for CSD
 lock diagnostics

On Tue, Apr 19, 2022 at 09:11:46AM +0200, Juergen Gross wrote:
> On 19.04.22 00:53, Paul E. McKenney wrote:
> > Debugging of problems involving insanely long-running SMI handlers
> > proceeds better if the CSD-lock timeout can be adjusted.  This commit
> > therefore provides a new smp.csd_lock_timeout kernel boot parameter
> > that specifies the timeout in milliseconds.  The default remains at the
> > previously hard-coded value of five seconds.
> > 
> > Cc: Rik van Riel <riel@...riel.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: Juergen Gross <jgross@...e.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >   Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++
> >   kernel/smp.c                                    |  7 +++++--
> >   2 files changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 3f1cc5e317ed..645c4c001b16 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5377,6 +5377,17 @@
> >   	smart2=		[HW]
> >   			Format: <io1>[,<io2>[,...,<io8>]]
> > +	smp.csd_lock_timeout= [KNL]
> > +			Specify the period of time in milliseconds
> > +			that smp_call_function() and friends will wait
> > +			for a CPU to release the CSD lock.  This is
> > +			useful when diagnosing bugs involving CPUs
> > +			disabling interrupts for extended periods
> > +			of time.  Defaults to 5,000 milliseconds, and
> > +			setting a value of zero disables this feature.
> > +			This feature may be more efficiently disabled
> > +			using the csdlock_debug- kernel parameter.
> > +
> >   	smsc-ircc2.nopnp	[HW] Don't use PNP to discover SMC devices
> >   	smsc-ircc2.ircc_cfg=	[HW] Device configuration I/O port
> >   	smsc-ircc2.ircc_sir=	[HW] SIR base I/O port
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 01a7c1706a58..d82439bac401 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -183,7 +183,9 @@ static DEFINE_PER_CPU(smp_call_func_t, cur_csd_func);
> >   static DEFINE_PER_CPU(void *, cur_csd_info);
> >   static DEFINE_PER_CPU(struct cfd_seq_local, cfd_seq_local);
> > -#define CSD_LOCK_TIMEOUT (5ULL * NSEC_PER_SEC)
> > +static ulong csd_lock_timeout = 5000;  /* CSD lock timeout in milliseconds. */
> > +module_param(csd_lock_timeout, ulong, 0444);
> > +
> >   static atomic_t csd_bug_count = ATOMIC_INIT(0);
> >   static u64 cfd_seq;
> > @@ -329,6 +331,7 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
> >   	u64 ts2, ts_delta;
> >   	call_single_data_t *cpu_cur_csd;
> >   	unsigned int flags = READ_ONCE(csd->node.u_flags);
> > +	unsigned long long csd_lock_timeout_ns = csd_lock_timeout * NSEC_PER_MSEC;
> >   	if (!(flags & CSD_FLAG_LOCK)) {
> >   		if (!unlikely(*bug_id))
> > @@ -341,7 +344,7 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
> >   	ts2 = sched_clock();
> >   	ts_delta = ts2 - *ts1;
> > -	if (likely(ts_delta <= CSD_LOCK_TIMEOUT))
> > +	if (likely(ts_delta <= csd_lock_timeout_ns || csd_lock_timeout_ns <= 0))
> 
> csd_lock_timeout_ns is unsigned, so "csd_lock_timeout_ns <= 0" should be rather
> "csd_lock_timeout_ns == 0".
> 
> With that fixed you can add my
> 
> Reviewed-by: Juergen Gross <jgross@...e.com>

Thank you, and will do on next rebase.

							Thanx, Paul