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Date:   Mon, 16 Oct 2023 17:52:57 -0300
From:   Leonardo Bras <leobras@redhat.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Imran Khan <imran.f.khan@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <vschneid@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH smp,csd] Throw an error if a CSD lock is stuck for too
 long
Message-ID: <ZS2iqck0tWDWEVMZ@redhat.com>
References: <bc7cc8b0-f587-4451-8bcd-0daae627bcc7@paulmck-laptop>
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On Mon, Oct 16, 2023 at 11:27:51AM -0700, Paul E. McKenney wrote:
> On Fri, Oct 13, 2023 at 12:26:22PM -0300, Leonardo Bras wrote:
> > On Mon, Oct 09, 2023 at 09:39:38AM -0700, Paul E. McKenney wrote:
> > > 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@surriel.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@surriel.com>
> > >     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > >     Cc: Peter Zijlstra <peterz@infradead.org>
> > >     Cc: Valentin Schneider <vschneid@redhat.com>
> > >     Cc: Juergen Gross <jgross@suse.com>
> > >     Cc: Jonathan Corbet <corbet@lwn.net>
> > >     Cc: Randy Dunlap <rdunlap@infradead.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.
> > > +
> > 
> > It could be interesting to have it as an s64 parameter (in {mili,}seconds) 
> > instead of bool, this way the user could pick the time to wait before the 
> > panic happens. 0 or -1 could mean disabled.
> > 
> > What do you think?
> > 
> > Other than that,
> > Reviewed-by: Leonardo Bras <leobras@redhat.com>
> 
> Thank you for looking this over!
> 
> How about with the diff shown below, to be folded into the original?
> I went with int instead of s64 because I am having some difficulty
> imagining anyone specifying more than a 24-day timeout.  ;-)

I suggested s64 just because it was the type being used in
BUG_ON(panic_on_ipistall && (s64)ts_delta > 300000000000LL);

But anyway, int should be fine.


> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ccb7621eff79..ea5ae9deb753 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5931,8 +5931,10 @@
>  
>  	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.
> +			the specified number of milliseconds, panic the
> +			system.  By default, let CSD-lock acquisition
> +			take as long as they take.  Specifying 300,000
> +			for this value provides a 10-minute timeout.

300,000 ms is 300s, which is 5 minutes, right?

>  
>  	smsc-ircc2.nopnp	[HW] Don't use PNP to discover SMC devices
>  	smsc-ircc2.ircc_cfg=	[HW] Device configuration I/O port
> diff --git a/kernel/smp.c b/kernel/smp.c
> index b6a0773a7015..d3ca47f32f38 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -170,8 +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 int panic_on_ipistall;  /* CSD panic timeout in milliseconds, 300000 for ten minutes. */

s/ten/five

> +module_param(panic_on_ipistall, int, 0444);
>  
>  static atomic_t csd_bug_count = ATOMIC_INIT(0);
>  
> @@ -256,7 +256,7 @@ static bool csd_lock_wait_toolong(struct __call_single_data *csd, u64 ts0, u64 *
>  	 * 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);
> +	BUG_ON(panic_on_ipistall > 0 && (s64)ts_delta > ((s64)panic_on_ipistall * NSEC_PER_MSEC));

s64 here would avoid casting (s64)panic_on_ipistall, but I think it does 
not really impact readability.  

IIUC ts_delta is an u64 being casted as s64, which could be an issue but no 
computer system will actually take over 2^31 ns (292 years) to run 1 
iteration, so it's safe.

I think it's a nice feaure :)

Thanks!
Leo


>  	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)),
>