LKML Archive on
help / color / mirror / Atom feed
From: Dexuan Cui <>
To: Ingo Molnar <>,
	Alexey Dobriyan <>,
	Andrew Morton <>,
	Peter Zijlstra <>,
	Thomas Gleixner <>,
	Greg Kroah-Hartman <>,
	Rakib Mullick <>
Cc: "''" <>
Subject: for_each_cpu()  is buggy for UP kernel?
Date: Wed, 9 May 2018 06:24:16 +0000	[thread overview]
Message-ID: <KL1P15301MB000678289FE55BA365B3279ABF990@KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM> (raw)

In include/linux/cpumask.h, for_each_cpu is defined like this for UP kernel (CONFIG_NR_CPUS=1):

#define for_each_cpu(cpu, mask)                 \
        for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)

Here 'mask' is ignored, but what if 'mask' contains 0 CPU? -- in this case, the for loop should not
run at all, but with the current code, we run the loop once with cpu==0.

I think I'm seeing a bug in my UP kernel that is caused by the buggy for_each_cpu():

in kernel/time/tick-broadcast.c: tick_handle_oneshot_broadcast(), tick_broadcast_oneshot_mask
contains 0 CPU, but due to the buggy for_each_cpu(), the variable 'next_event' is changed from
its default value KTIME_MAX to "next_event = td->evtdev->next_event"; as a result,
tick_handle_oneshot_broadcast () -> tick_broadcast_set_event() -> clockevents_program_event()
-> pit_next_event() is programming the PIT timer by accident, causing an interrupt storm of PIT
interrupts in some way: I'm seeing that the kernel is receiving ~8000 PIT interrupts per second for
1~5 minutes when the UP kernel boots, and it looks the kernel hangs, but in 1~5 minutes, finally
somehow the kernel can recover and boot up fine. But, occasionally, the kernel just hangs there
forever, receiving ~8000 PIT timers per second. 

With the below change in kernel/time/tick-broadcast.c, the interrupt storm will go away:

+#undef for_each_cpu
+#define for_each_cpu(cpu, mask)                        \
+       for ((cpu) = 0; (((cpu) < 1) && ((mask)[0].bits[0] & 1)); (cpu)++, (void)mask)

Should we fix the for_each_cpu() in include/linux/cpumask.h for UP?

-- Dexuan

             reply	other threads:[~2018-05-09  6:24 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-09  6:24 Dexuan Cui [this message]
2018-05-09 23:20 ` Andrew Morton
2018-05-13 13:35   ` Thomas Gleixner
2018-05-13 18:21 ` Linus Torvalds
2018-05-14  7:28   ` Dmitry Vyukov
2018-05-15  3:02   ` Dexuan Cui
2018-05-15 17:21     ` Linus Torvalds
2018-05-15 20:10       ` Dexuan Cui
2018-05-15 19:52 ` [PATCH] tick/broadcast: Use for_each_cpu() specially on UP kernels Dexuan Cui
2018-05-15 20:48   ` [tip:timers/urgent] " tip-bot for Dexuan Cui

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=KL1P15301MB000678289FE55BA365B3279ABF990@KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM \ \ \ \ \ \ \ \ \ \
    --subject='Re: for_each_cpu()  is buggy for UP kernel?' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).