Re: [PATCH v2] trace: Schedule a delayed work to call wakeup()

[Steven Rostedt]

On Tue, 2011-05-10 at 13:27 -0700, Vaibhav Nagarnaik wrote:
> In using syscall tracing by concurrent processes, the wakeup() that is
> called in the event commit function causes contention on the spin lock
> of the waitqueue. I enabled sys_enter_getuid and sys_exit_getuid
> tracepoints, and by running getuid_microbench from autotest in parallel
> I found that the contention causes exponential latency increase in the
> tracing path.
> 
> The autotest binary getuid_microbench calls getuid() in a tight loop for
> the given number of iterations and measures the average time required to
> complete a single invocation of syscall.
> 
> The patch schedules a delayed work after 2 ms once an event commit calls
> to wake up the trace wait_queue. This removes the delay caused by
> contention on spin lock in wakeup() and amortizes the wakeup() calls
> scheduled over the 2 ms period.
> 
> In the following example, the script enables the sys_enter_getuid and
> sys_exit_getuid tracepoints and runs the getuid_microbench in parallel
> with the given number of processes. The output clearly shows the latency
> increase caused by contentions.
> 

Have you taken a look at irq_work() instead of delayed work? This would
probably allow us to call the wakeup version for all events.

See kernel/perf_events.c for details.

-- Steve

> $ ~/getuid.sh 1
> 1000000 calls in 0.720974253 s (720.974253 ns/call)
> 
> $ ~/getuid.sh 2
> 1000000 calls in 1.166457554 s (1166.457554 ns/call)
> 1000000 calls in 1.168933765 s (1168.933765 ns/call)
> 
> $ ~/getuid.sh 3
> 1000000 calls in 1.783827516 s (1783.827516 ns/call)
> 1000000 calls in 1.795553270 s (1795.553270 ns/call)
> 1000000 calls in 1.796493376 s (1796.493376 ns/call)
> 
> $ ~/getuid.sh 4
> 1000000 calls in 4.483041796 s (4483.041796 ns/call)
> 1000000 calls in 4.484165388 s (4484.165388 ns/call)
> 1000000 calls in 4.484850762 s (4484.850762 ns/call)
> 1000000 calls in 4.485643576 s (4485.643576 ns/call)
> 
> $ ~/getuid.sh 5
> 1000000 calls in 6.497521653 s (6497.521653 ns/call)
> 1000000 calls in 6.502000236 s (6502.000236 ns/call)
> 1000000 calls in 6.501709115 s (6501.709115 ns/call)
> 1000000 calls in 6.502124100 s (6502.124100 ns/call)
> 1000000 calls in 6.502936358 s (6502.936358 ns/call)
> 
> After the patch, the latencies scale better.
> 1000000 calls in 0.728720455 s (728.720455 ns/call)
> 
> 1000000 calls in 0.842782857 s (842.782857 ns/call)
> 1000000 calls in 0.883803135 s (883.803135 ns/call)
> 
> 1000000 calls in 0.902077764 s (902.077764 ns/call)
> 1000000 calls in 0.902838202 s (902.838202 ns/call)
> 1000000 calls in 0.908896885 s (908.896885 ns/call)
> 
> 1000000 calls in 0.932523515 s (932.523515 ns/call)
> 1000000 calls in 0.958009672 s (958.009672 ns/call)
> 1000000 calls in 0.986188020 s (986.188020 ns/call)
> 1000000 calls in 0.989771102 s (989.771102 ns/call)
> 
> 1000000 calls in 0.933518391 s (933.518391 ns/call)
> 1000000 calls in 0.958897947 s (958.897947 ns/call)
> 1000000 calls in 1.031038897 s (1031.038897 ns/call)
> 1000000 calls in 1.089516025 s (1089.516025 ns/call)
> 1000000 calls in 1.141998347 s (1141.998347 ns/call)
> 
> Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@xxxxxxxxxx>
> ---
>  kernel/trace/trace.c |   24 +++++++++++++-----------
>  1 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 51e4e4d..c72d150 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -343,26 +343,28 @@ unsigned long trace_flags = TRACE_ITER_PRINT_PARENT | TRACE_ITER_PRINTK |
>  static int trace_stop_count;
>  static DEFINE_SPINLOCK(tracing_start_lock);
>  
> +static atomic_t scheduled = {0};
> +static void wakeup_work_handler(struct work_struct *work)
> +{
> +	wake_up(&trace_wait);
> +}
> +
> +static DECLARE_DELAYED_WORK(wakeup_work, wakeup_work_handler);
> +
>  /**
>   * trace_wake_up - wake up tasks waiting for trace input
>   *
> - * Simply wakes up any task that is blocked on the trace_wait
> - * queue. These is used with trace_poll for tasks polling the trace.
> + * Schedules a delayed work to wake up any task that is blocked on the
> + * trace_wait queue. These is used with trace_poll for tasks polling the
> + * trace.
>   */
>  void trace_wake_up(void)
>  {
> -	int cpu;
> +	const unsigned long delay = msecs_to_jiffies(2); //delay 2 ms
>  
>  	if (trace_flags & TRACE_ITER_BLOCK)
>  		return;
> -	/*
> -	 * The runqueue_is_locked() can fail, but this is the best we
> -	 * have for now:
> -	 */
> -	cpu = get_cpu();
> -	if (!runqueue_is_locked(cpu))
> -		wake_up(&trace_wait);
> -	put_cpu();
> +	schedule_delayed_work(&wakeup_work, delay);
>  }
>  
>  static int __init set_buf_size(char *str)


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