linux-kernel - Re: [PATCH] abort secondary CPU bring-up gracefully if do_boot_cpu timed out on cpu_callin

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Message-ID: <20140306101009.34ba6c3c@nial.usersys.redhat.com>
Date:	Thu, 6 Mar 2014 10:10:09 +0100
From:	Igor Mammedov <imammedo@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, drjones@...hat.com,
	toshi.kani@...com, prarit@...hat.com,
	kirill.shutemov@...ux.intel.com, JBeulich@...e.com,
	peterz@...radead.org, paul.gortmaker@...driver.com,
	seiji.aguchi@....com, bp@...e.de, hpa@...or.com, mingo@...hat.com,
	x86@...nel.org, tglx@...utronix.de, riel@...hat.com
Subject: Re: [PATCH] abort secondary CPU bring-up gracefully if do_boot_cpu
 timed out on cpu_callin_mask

On Thu, 6 Mar 2014 08:08:32 +0100
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Igor Mammedov <imammedo@...hat.com> wrote:
> 
> > Master CPU may timeout before cpu_callin_mask is set and cancel
> > booting CPU, but being onlined CPU still continues to boot, sets
> > cpu_active_mask (CPU_STARTING notifiers) and spins in
> > check_tsc_sync_target() for master cpu to arrive. Following attempt
> > to online another cpu hangs in stop_machine, initiated from here:
> 
> The changelog needs to prominently contain a description of the 
> practical relevance of this patch: has the hang triggered on any 
> system and under what circumstances, and did the patch resolve the 
> hang, etc.?

Hang is observed on virtual machines during CPU hotplug,
especially in big guests with many CPUs. (It happens more
often if host is over-committed).

Similar patch is carried in RHEL6 since 2012 and it fixes
issue there, when applied to upstream kernel it also fixes
issue.

> 
> Thanks,
> 
> 	Ingo

Thanks,
  Igor.
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