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    SubjectRe: [BUG] using smp_processor_id() in preemptible
    On Sun 16-11-08 08:10:59, Ingo Molnar wrote:
    >
    > * Luis Henriques <henrix@sapo.pt> wrote:
    >
    > > Hi Ingo,
    > >
    > > while running latencytop, got the following:
    > >
    > > [ 775.663239] BUG: using smp_processor_id() in preemptible [00000000] code: latencytop/6585
    > > [ 775.663303] caller is native_sched_clock+0x3a/0x80
    > > [ 775.663314] Pid: 6585, comm: latencytop Tainted: G W 2.6.28-rc4-00355-g9c7c354 #1
    > > [ 775.663322] Call Trace:
    > > [ 775.663343] [<ffffffff803a94e4>] debug_smp_processor_id+0xe4/0xf0
    > > [ 775.663356] [<ffffffff80213f7a>] native_sched_clock+0x3a/0x80
    > > [ 775.663368] [<ffffffff80213e19>] sched_clock+0x9/0x10
    > > [ 775.663381] [<ffffffff8024550d>] proc_sched_show_task+0x8bd/0x10e0
    > > [ 775.663395] [<ffffffff8034466e>] sched_show+0x3e/0x80
    > > [ 775.663408] [<ffffffff8031039b>] seq_read+0xdb/0x350
    > > [ 775.663421] [<ffffffff80368776>] ? security_file_permission+0x16/0x20
    > > [ 775.663435] [<ffffffff802f4198>] vfs_read+0xc8/0x170
    > > [ 775.663447] [<ffffffff802f4335>] sys_read+0x55/0x90
    > > [ 775.663460] [<ffffffff8020c67a>] system_call_fastpath+0x16/0x1b
    > > ...
    > >
    > > I am running 2.6.28-rc4-00355-g9c7c354 and, after taking a look at
    > > the code, it looks like the problem is in commit
    > > 7cbaef9c83e58bbd4bdd534b09052b6c5ec457d5, where native_sched_clock
    > > is modified to use __cycles_2_ns instead of cycles_2_ns.
    >
    > indeed. Does the fix below work for you?

    Yes, it fixes problem on my end... (reported at
    http://www.gossamer-threads.com/lists/linux/kernel/997205).

    >
    > Thanks,
    >
    > Ingo
    >
    > -------------->
    > From 29d7b90c15035741d15421b36000509212b3e135 Mon Sep 17 00:00:00 2001
    > From: Ingo Molnar <mingo@elte.hu>
    > Date: Sun, 16 Nov 2008 08:07:15 +0100
    > Subject: [PATCH] sched: fix kernel warning on /proc/sched_debug access
    >
    > Luis Henriques reported that with CONFIG_PREEMPT=y + CONFIG_PREEMPT_DEBUG=y +
    > CONFIG_SCHED_DEBUG=y + CONFIG_LATENCYTOP=y enabled, the following warning
    > triggers when using latencytop:
    >
    > > [ 775.663239] BUG: using smp_processor_id() in preemptible [00000000] code: latencytop/6585
    > > [ 775.663303] caller is native_sched_clock+0x3a/0x80
    > > [ 775.663314] Pid: 6585, comm: latencytop Tainted: G W 2.6.28-rc4-00355-g9c7c354 #1
    > > [ 775.663322] Call Trace:
    > > [ 775.663343] [<ffffffff803a94e4>] debug_smp_processor_id+0xe4/0xf0
    > > [ 775.663356] [<ffffffff80213f7a>] native_sched_clock+0x3a/0x80
    > > [ 775.663368] [<ffffffff80213e19>] sched_clock+0x9/0x10
    > > [ 775.663381] [<ffffffff8024550d>] proc_sched_show_task+0x8bd/0x10e0
    > > [ 775.663395] [<ffffffff8034466e>] sched_show+0x3e/0x80
    > > [ 775.663408] [<ffffffff8031039b>] seq_read+0xdb/0x350
    > > [ 775.663421] [<ffffffff80368776>] ? security_file_permission+0x16/0x20
    > > [ 775.663435] [<ffffffff802f4198>] vfs_read+0xc8/0x170
    > > [ 775.663447] [<ffffffff802f4335>] sys_read+0x55/0x90
    > > [ 775.663460] [<ffffffff8020c67a>] system_call_fastpath+0x16/0x1b
    > > ...
    >
    > This breakage was caused by me via:
    >
    > 7cbaef9: sched: optimize sched_clock() a bit
    >
    > Change the calls to cpu_clock().
    >
    > Reported-by: Luis Henriques <henrix@sapo.pt>

    Tested-by: Michal Hocko <mhocko@suse.cz>

    > ---
    > kernel/sched_debug.c | 5 +++--
    > 1 files changed, 3 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
    > index 48ecc51..26ed8e3 100644
    > --- a/kernel/sched_debug.c
    > +++ b/kernel/sched_debug.c
    > @@ -423,10 +423,11 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
    > #undef __P
    >
    > {
    > + unsigned int this_cpu = raw_smp_processor_id();
    > u64 t0, t1;
    >
    > - t0 = sched_clock();
    > - t1 = sched_clock();
    > + t0 = cpu_clock(this_cpu);
    > + t1 = cpu_clock(this_cpu);
    > SEQ_printf(m, "%-35s:%21Ld\n",
    > "clock-delta", (long long)(t1-t0));
    > }
    >
    > --
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    --
    Michal Hocko
    L3 team
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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