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    SubjectRe: [PATCH] tracing/ring-buffer: fix non cpu hotplug case

    On Thu, 19 Mar 2009, Frederic Weisbecker wrote:

    > Impact: fix warning with irqsoff tracer
    >
    > The ring buffer allocates its buffers on pre-smp time (early_initcall).
    > It means that, at first, only the boot cpu buffer is allocated and
    > the ring-buffer cpumask only has the boot cpu set (cpu_online_mask).
    >
    > Later, the secondary cpu will show up and the ring-buffer will be notified
    > about this event: the appropriate buffer will be allocated and the cpumask
    > will be updated.
    >
    > Unfortunately, if !CONFIG_CPU_HOTPLUG, the ring-buffer will not be
    > notified about the secondary cpus, meaning that the cpumask will have
    > only the cpu boot set, and only one cpu buffer allocated.
    >
    > We fix that by using cpu_possible_mask if !CONFIG_CPU_HOTPLUG.
    >
    > This patch fixes the following warning with irqsoff tracer running:
    >
    > [ 169.317794] WARNING: at kernel/trace/trace.c:466 update_max_tr_single+0xcc/0xf3()
    > [ 169.318002] Hardware name: AMILO Li 2727
    > [ 169.318002] Modules linked in:
    > [ 169.318002] Pid: 5624, comm: bash Not tainted 2.6.29-rc8-tip-02636-g6aafa6c #11
    > [ 169.318002] Call Trace:
    > [ 169.318002] [<ffffffff81036182>] warn_slowpath+0xea/0x13d
    > [ 169.318002] [<ffffffff8100b9d6>] ? ftrace_call+0x5/0x2b
    > [ 169.318002] [<ffffffff8100b9d6>] ? ftrace_call+0x5/0x2b
    > [ 169.318002] [<ffffffff8100b9d1>] ? ftrace_call+0x0/0x2b
    > [ 169.318002] [<ffffffff8101ef10>] ? ftrace_modify_code+0xa9/0x108
    > [ 169.318002] [<ffffffff8106e27f>] ? trace_hardirqs_off+0x25/0x27
    > [ 169.318002] [<ffffffff8149afe7>] ? _spin_unlock_irqrestore+0x1f/0x2d
    > [ 169.318002] [<ffffffff81064f52>] ? ring_buffer_reset_cpu+0xf6/0xfb
    > [ 169.318002] [<ffffffff8106637c>] ? ring_buffer_reset+0x36/0x48
    > [ 169.318002] [<ffffffff8106aeda>] update_max_tr_single+0xcc/0xf3
    > [ 169.318002] [<ffffffff8100bc17>] ? sysret_check+0x22/0x5d
    > [ 169.318002] [<ffffffff8106e3ea>] stop_critical_timing+0x142/0x204
    > [ 169.318002] [<ffffffff8106e4cf>] trace_hardirqs_on_caller+0x23/0x25
    > [ 169.318002] [<ffffffff8149ac28>] trace_hardirqs_on_thunk+0x3a/0x3c
    > [ 169.318002] [<ffffffff8100bc17>] ? sysret_check+0x22/0x5d
    > [ 169.318002] ---[ end trace db76cbf775a750cf ]---
    >
    > Because this tracer may try to swap two cpu ring buffers for an
    > unregistered cpu on the ring buffer.
    >
    > This patch might also fix a fair loss of traces due to unallocated buffers
    > for secondary cpus.

    Thanks Frederic!

    Acked-by: Steven Rostedt <srostedt@redhat.com>

    -- Steve

    >
    > Reported-by: Ingo Molnar <mingo@elte.hu>
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > ---
    > kernel/trace/ring_buffer.c | 9 +++++++++
    > 1 files changed, 9 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
    > index c9ca8a8..bc74f00 100644
    > --- a/kernel/trace/ring_buffer.c
    > +++ b/kernel/trace/ring_buffer.c
    > @@ -578,8 +578,17 @@ struct ring_buffer *ring_buffer_alloc(unsigned long size, unsigned flags)
    > if (buffer->pages == 1)
    > buffer->pages++;
    >
    > + /*
    > + * In case of non-hotplug cpu, if the ring-buffer is allocated
    > + * in early initcall, it will not be notified of secondary cpus.
    > + * In that off case, we need to allocate for all possible cpus.
    > + */
    > +#ifdef CONFIG_HOTPLUG_CPU
    > get_online_cpus();
    > cpumask_copy(buffer->cpumask, cpu_online_mask);
    > +#else
    > + cpumask_copy(buffer->cpumask, cpu_possible_mask);
    > +#endif
    > buffer->cpus = nr_cpu_ids;
    >
    > bsize = sizeof(void *) * nr_cpu_ids;
    > --
    > 1.6.1
    >
    >


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