linux-kernel - Re: ftrace hangs waiting for rcu

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Message-ID: <yt9da6ffn8bn.fsf@linux.ibm.com>
Date:   Fri, 28 Jan 2022 17:17:32 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yinan Liu <yinan@...ux.alibaba.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org, hca@...ux.ibm.com,
        linux-s390@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: ftrace hangs waiting for rcu

Hi Mark,

Mark Rutland <mark.rutland@....com> writes:

> On Fri, Jan 28, 2022 at 05:08:48PM +0100, Sven Schnelle wrote:
>> We noticed the PR from Paul and are currently testing the fix. So far
>> it's looking good. The configuration where we have seen the hang is a
>> bit unusual:
>> 
>> - 16 physical CPUs on the kvm host
>> - 248 logical CPUs inside kvm
>
> Aha! 248 is notably *NOT* a power of two, and in this case the shift would be
> wrong (ilog2() would give 7, when we need a shift of 8).
>
> So I suspect you're hitting the same issue as I was.

Argh, indeed! I somehow changed 'power of two' to 'odd number' in my
head. I guess it's time for the weekend. :-)

Thanks!