linux-kernel - Re: [tip:smp/hotplug] cpu/hotplug: Abort disabling secondary CPUs if wakeup is pending

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Date:   Mon, 30 Mar 2020 19:32:06 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     tglx@...utronix.de, peterz@...radead.org,
        linux-kernel@...r.kernel.org, len.brown@...el.com,
        pkondeti@...eaurora.org, jpoimboe@...hat.com, pavel@....cz,
        konrad.wilk@...cle.com, mojha@...eaurora.org, jkosina@...e.cz,
        mingo@...nel.org, hpa@...or.com, rjw@...ysocki.net,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:smp/hotplug] cpu/hotplug: Abort disabling secondary CPUs if
 wakeup is pending

On 03/27/20 10:53, Boqun Feng wrote:
> Hi Thomas and Pavankumar,
> 
> I have a question about this patch, please see below:
> 
> On Wed, Jun 12, 2019 at 05:34:08AM -0700, tip-bot for Pavankumar Kondeti wrote:
> > Commit-ID:  a66d955e910ab0e598d7a7450cbe6139f52befe7
> > Gitweb:     https://git.kernel.org/tip/a66d955e910ab0e598d7a7450cbe6139f52befe7
> > Author:     Pavankumar Kondeti <pkondeti@...eaurora.org>
> > AuthorDate: Mon, 3 Jun 2019 10:01:03 +0530
> > Committer:  Thomas Gleixner <tglx@...utronix.de>
> > CommitDate: Wed, 12 Jun 2019 11:03:05 +0200
> > 
> > cpu/hotplug: Abort disabling secondary CPUs if wakeup is pending
> > 
> > When "deep" suspend is enabled, all CPUs except the primary CPU are frozen
> > via CPU hotplug one by one. After all secondary CPUs are unplugged the
> > wakeup pending condition is evaluated and if pending the suspend operation
> > is aborted and the secondary CPUs are brought up again.
> > 
> > CPU hotplug is a slow operation, so it makes sense to check for wakeup
> > pending in the freezer loop before bringing down the next CPU. This
> > improves the system suspend abort latency significantly.
> > 
> 
> From the commit message, it makes sense to add the pm_wakeup_pending()
> check if freeze_secondary_cpus() is used for system suspend. However,
> freeze_secondary_cpus() is also used in kexec path on arm64:
> 
> 	kernel_kexec():
> 	  machine_shutdown():
> 	    disable_nonboot_cpus():
> 	      freeze_secondary_cpus()

FWIW, I fixed this already and the change was picked up:

	https://lore.kernel.org/lkml/20200323135110.30522-7-qais.yousef@arm.com/

Only x86 now uses disable_nonboot_cpus().

# tip/smp/core

$ git grep disable_nonboot_cpus
Documentation/power/suspend-and-cpuhotplug.rst:                              disable_nonboot_cpus()
Documentation/power/suspend-and-cpuhotplug.rst:                       /* disable_nonboot_cpus() complete */
arch/x86/power/cpu.c:   ret = disable_nonboot_cpus();
include/linux/cpu.h:static inline int disable_nonboot_cpus(void)
include/linux/cpu.h:static inline int disable_nonboot_cpus(void) { return 0; }
kernel/cpu.c:    * this even in case of failure as all disable_nonboot_cpus() users are

Thanks

--
Qais Yousef

> 
> , so I wonder whether the pm_wakeup_pending() makes sense in this
> situation? Because IIUC, in this case we want to reboot the system
> regardlessly, the pm_wakeup_pending() checking seems to be inappropriate
> then.
> 
> I'm asking this because I'm debugging a kexec failure on ARM64 guest on
> Hyper-V, and I got the BUG_ON() triggered:
> 
> [  108.378016] kexec_core: Starting new kernel
> [  108.378018] Disabling non-boot CPUs ...
> [  108.378019] Wakeup pending. Abort CPU freeze
> [  108.378020] Non-boot CPUs are not disabled
> [  108.378049] ------------[ cut here ]------------
> [  108.378050] kernel BUG at arch/arm64/kernel/machine_kexec.c:154!
> 
> Thanks!
> 
> Regards,
> Boqun
> 
> > [ tglx: Massaged changelog and improved printk message ]
> > 
> > Signed-off-by: Pavankumar Kondeti <pkondeti@...eaurora.org>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> > Cc: Len Brown <len.brown@...el.com>
> > Cc: Pavel Machek <pavel@....cz>
> > Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > Cc: iri Kosina <jkosina@...e.cz>
> > Cc: Mukesh Ojha <mojha@...eaurora.org>
> > Cc: linux-pm@...r.kernel.org
> > Link: https://lkml.kernel.org/r/1559536263-16472-1-git-send-email-pkondeti@codeaurora.org
> > 
> > ---
> >  kernel/cpu.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index be82cbc11a8a..0778249cd49d 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -1221,6 +1221,13 @@ int freeze_secondary_cpus(int primary)
> >  	for_each_online_cpu(cpu) {
> >  		if (cpu == primary)
> >  			continue;
> > +
> > +		if (pm_wakeup_pending()) {
> > +			pr_info("Wakeup pending. Abort CPU freeze\n");
> > +			error = -EBUSY;
> > +			break;
> > +		}
> > +
> >  		trace_suspend_resume(TPS("CPU_OFF"), cpu, true);
> >  		error = _cpu_down(cpu, 1, CPUHP_OFFLINE);
> >  		trace_suspend_resume(TPS("CPU_OFF"), cpu, false);