[PATCH v4 2/3] trace: process entire stop_critical_timing() on the same CPU

[Petr Mladek]

The interrupts off latency tracing heavily depends on disabled interrupts.
This is also the case of stop_critical_timings(). There is the following
nested call:

  + stop_critical_timing()
    + check_critical_timing()
      + update_max_tr_single()
        + ring_buffer_swap_cpu()

, where ring_buffer_swap_cpu() works with disabled interrupts only when
the swap is done for this cpu.

This patch solves the problem by disabling preemption early in
stop_critical_timings() and doing the whole job on the same CPU.

Note that big part was done with disabled preemption even before.
See raw_spin_lock_irqsave() in check_critical_timing(). The extended
scope affects only few fast operations and should not cause any harm
to the system.

Signed-off-by: Petr Mladek <pmla...@suse.cz>
---
 kernel/trace/trace_irqsoff.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 9bb104f748d0..1d4eeb304583 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -405,21 +405,26 @@ stop_critical_timing(unsigned long ip, unsigned long 
parent_ip)
        struct trace_array_cpu *data;
        unsigned long flags;
 
-       cpu = raw_smp_processor_id();
+       /*
+        * We need to call ring_buffer_swap_cpu() with disabled interrupts.
+        * Let's make sure that it works with this CPU buffer.
+        */
+       cpu = get_cpu();
+
        /* Always clear the tracing cpu on stopping the trace */
        if (unlikely(per_cpu(tracing_cpu, cpu)))
                per_cpu(tracing_cpu, cpu) = 0;
        else
-               return;
+               goto out;
 
        if (!tracer_enabled || !tracing_is_enabled())
-               return;
+               goto out;
 
        data = per_cpu_ptr(tr->trace_buffer.data, cpu);
 
        if (unlikely(!data) ||
            !data->critical_start || atomic_read(&data->disabled))
-               return;
+               goto out;
 
        atomic_inc(&data->disabled);
 
@@ -428,6 +433,8 @@ stop_critical_timing(unsigned long ip, unsigned long 
parent_ip)
        check_critical_timing(tr, data, parent_ip ? : ip, cpu);
        data->critical_start = 0;
        atomic_dec(&data->disabled);
+out:
+       put_cpu();
 }
 
 /* start and stop critical timings used to for stoppage (in idle) */
-- 
1.8.4

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