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Date: Fri, 10 May 2013 17:21:02 +0200
From: Borislav Petkov <bp@alien8.de>
To: Jiri Kosina <jkosina@suse.cz>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, Tony Luck <tony.luck@intel.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123
 native_smp_send_reschedule
Message-ID: <20130510152102.GD22942@pd.tnic>
References: <alpine.LNX.2.00.1305091423460.23038@pobox.suse.cz>
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On Fri, May 10, 2013 at 05:03:56PM +0200, Jiri Kosina wrote:
> [ ... snip ... ]
>  Enabling non-boot CPUs ...
>  smpboot: Booting Node 0 Processor 1 APIC 0x1
>  CPU1 microcode updated early to revision 0x60f, date = 2010-09-29
>  Disabled fast string operations
>  1 1
>  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.9.0-12317-gb2031d4 #1
>  Hardware name: LENOVO 7470BN2/7470BN2, BIOS 6DET38WW (2.02 ) 12/19/2008
>   ffff88007c28cca0 ffff880079851e08 ffffffff8154837e ffff880079851e28
>   ffffffff81077514 ffff88007c28cca0 ffff88007c28cca0 ffff880079851e68
>   ffffffff810529db 0000000179851e78 ffff88007c28cca0 0000000000000001
>  Call Trace:
>   [<ffffffff8154837e>] dump_stack+0x19/0x1b
>   [<ffffffff81077514>] wake_up_nohz_cpu+0xd4/0xf0
>   [<ffffffff810529db>] add_timer_on+0xdb/0x110
>   [<ffffffff8101e4f4>] mce_start_timer+0x64/0x70
>   [<ffffffff8101e552>] __mcheck_cpu_init_timer+0x52/0x60
>   [<ffffffff8153e22e>] mcheck_cpu_init+0x6f/0x111
>   [<ffffffff8153b94e>] identify_cpu+0x3cc/0x3f9
>   [<ffffffff8153b98d>] identify_secondary_cpu+0x12/0x1d
>   [<ffffffff8153fdd6>] smp_store_cpu_info+0x3a/0x3c
>   [<ffffffff8153fec2>] smp_callin+0xea/0x1c1
>   [<ffffffff8153ffbd>] start_secondary+0x24/0x97

Ok, I got it:

smp_callin is called by start_secondary() and down that path we add the
timer and do wake_up_nohz_cpu.

HOWEVER(!), the bit in the cpu_online_mask is set much later in
smp_callin() with

	set_cpu_online(smp_processor_id(), true);

Thus, when we come to send the IPI, the cpu is still offline, according
to the cpu_online_mask, thus the WARN_ON.

Nice :-\

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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