Date:   Mon, 9 Oct 2023 09:39:38 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Imran Khan <imran.f.khan@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Leonardo Bras <leobras@...hat.com>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH smp,csd] Throw an error if a CSD lock is stuck for too
 long

On Fri, Oct 06, 2023 at 10:32:07AM +1100, Imran Khan wrote:
> Hello Paul,
> 
> On 6/10/2023 3:48 am, Paul E. McKenney wrote:
> > The CSD lock seems to get stuck in 2 "modes". When it gets stuck
> > temporarily, it usually gets released in a few seconds, and sometimes
> > up to one or two minutes.
> > 
> > If the CSD lock stays stuck for more than several minutes, it never
> > seems to get unstuck, and gradually more and more things in the system
> > end up also getting stuck.
> > 
> > In the latter case, we should just give up, so the system can dump out
> > a little more information about what went wrong, and, with panic_on_oops
> > and a kdump kernel loaded, dump a whole bunch more information about
> > what might have gone wrong.
> > 
> > Question: should this have its own panic_on_ipistall switch in
> > /proc/sys/kernel, or maybe piggyback on panic_on_oops in a different
> > way than via BUG_ON?
> > 
> panic_on_ipistall (set to 1 by default) looks better option to me. For systems
> where such delay is acceptable and system can eventually get back to sane state,
> this option (set to 0 after boot) would prevent crashing the system for
> apparently benign CSD hangs of long duration.

Good point!  How about like the following?

							Thanx, Paul

------------------------------------------------------------------------

commit 6bcf3786291b86f13b3e13d51e998737a8009ec3
Author: Rik van Riel <riel@...riel.com>
Date:   Mon Aug 21 16:04:09 2023 -0400

    smp,csd: Throw an error if a CSD lock is stuck for too long
    
    The CSD lock seems to get stuck in 2 "modes". When it gets stuck
    temporarily, it usually gets released in a few seconds, and sometimes
    up to one or two minutes.
    
    If the CSD lock stays stuck for more than several minutes, it never
    seems to get unstuck, and gradually more and more things in the system
    end up also getting stuck.
    
    In the latter case, we should just give up, so the system can dump out
    a little more information about what went wrong, and, with panic_on_oops
    and a kdump kernel loaded, dump a whole bunch more information about what
    might have gone wrong.  In addition, there is an smp.panic_on_ipistall
    kernel boot parameter that by default retains the old behavior, but when
    set enables the panic after the CSD lock has been stuck for more than
    five minutes.
    
    [ paulmck: Apply Imran Khan feedback. ]
    
    Link: https://lore.kernel.org/lkml/bc7cc8b0-f587-4451-8bcd-0daae627bcc7@paulmck-laptop/
    Signed-off-by: Rik van Riel <riel@...riel.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
    Cc: Peter Zijlstra <peterz@...radead.org>
    Cc: Valentin Schneider <vschneid@...hat.com>
    Cc: Juergen Gross <jgross@...e.com>
    Cc: Jonathan Corbet <corbet@....net>
    Cc: Randy Dunlap <rdunlap@...radead.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0a1731a0f0ef..592935267ce2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5858,6 +5858,11 @@
 			This feature may be more efficiently disabled
 			using the csdlock_debug- kernel parameter.
 
+	smp.panic_on_ipistall= [KNL]
+			If a csd_lock_timeout extends for more than
+			five minutes, panic the system.  By default, let
+			CSD-lock acquisition take as long as they take.
+
 	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 8455a53465af..b6a0773a7015 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -170,6 +170,8 @@ static DEFINE_PER_CPU(void *, cur_csd_info);
 
 static ulong csd_lock_timeout = 5000;  /* CSD lock timeout in milliseconds. */
 module_param(csd_lock_timeout, ulong, 0444);
+static bool panic_on_ipistall;
+module_param(panic_on_ipistall, bool, 0444);
 
 static atomic_t csd_bug_count = ATOMIC_INIT(0);
 
@@ -230,6 +232,7 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
 	}
 
 	ts2 = sched_clock();
+	/* How long since we last checked for a stuck CSD lock.*/
 	ts_delta = ts2 - *ts1;
 	if (likely(ts_delta <= csd_lock_timeout_ns || csd_lock_timeout_ns == 0))
 		return false;
@@ -243,9 +246,17 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
 	else
 		cpux = cpu;
 	cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux)); /* Before func and info. */
+	/* How long since this CSD lock was stuck. */
+	ts_delta = ts2 - ts0;
 	pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %llu ns for CPU#%02d %pS(%ps).\n",
-		 firsttime ? "Detected" : "Continued", *bug_id, raw_smp_processor_id(), ts2 - ts0,
+		 firsttime ? "Detected" : "Continued", *bug_id, raw_smp_processor_id(), ts_delta,
 		 cpu, csd->func, csd->info);
+	/*
+	 * If the CSD lock is still stuck after 5 minutes, it is unlikely
+	 * to become unstuck. Use a signed comparison to avoid triggering
+	 * on underflows when the TSC is out of sync between sockets.
+	 */
+	BUG_ON(panic_on_ipistall && (s64)ts_delta > 300000000000LL);
 	if (cpu_cur_csd && csd != cpu_cur_csd) {
 		pr_alert("\tcsd: CSD lock (#%d) handling prior %pS(%ps) request.\n",
 			 *bug_id, READ_ONCE(per_cpu(cur_csd_func, cpux)),

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