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Date: Tue, 17 Feb 2009 23:55:39 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org,
Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH 1/2 v2] tracing/core: use appropriate waiting on
trace_pipe
On Wed, Feb 11, 2009 at 02:25:00AM +0100, Frederic Weisbecker wrote:
> Impact: api and pipe waiting change
>
> Currently, the waiting used in tracing_read_pipe() is done through a
> 100 msecs schedule_timeout() loop which periodically check if there
> are traces on the buffer.
>
> This can cause small latencies for programs which are reading the incoming
> events.
>
> This patch makes the reader waiting for the trace_wait waitqueue except
> for few tracers such as the sched and functions tracers which might be
> already hold the runqueue lock while waking up the reader.
>
> This is performed through a new callback wait_pipe() on struct tracer.
> If none is implemented on a specific tracer, the default waiting for
> trace_wait queue is attached.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
> kernel/trace/trace.c | 62 ++++++++++++++++++++++-----------
> kernel/trace/trace.h | 25 ++++++++++++--
> kernel/trace/trace_functions.c | 1 +
> kernel/trace/trace_functions_graph.c | 1 +
> kernel/trace/trace_sched_switch.c | 1 +
> kernel/trace/trace_sched_wakeup.c | 1 +
> 6 files changed, 67 insertions(+), 24 deletions(-)
Steven, Ingo, do you agree this one?
Thanks.
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d7c175a..9c3b684 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -497,6 +497,9 @@ int register_tracer(struct tracer *type)
> else
> if (!type->flags->opts)
> type->flags->opts = dummy_tracer_opt;
> + if (!type->wait_pipe)
> + type->wait_pipe = default_wait_pipe;
> +
>
> #ifdef CONFIG_FTRACE_STARTUP_TEST
> if (type->selftest && !tracing_selftest_disabled) {
> @@ -1060,7 +1063,10 @@ tracing_sched_wakeup_trace(struct trace_array *tr,
> entry->next_prio = wakee->prio;
> entry->next_state = wakee->state;
> entry->next_cpu = task_cpu(wakee);
> - trace_buffer_unlock_commit(tr, event, flags, pc);
> +
> + ring_buffer_unlock_commit(tr->buffer, event);
> + ftrace_trace_stack(tr, flags, 6, pc);
> + ftrace_trace_userstack(tr, flags, pc);
> }
>
> void
> @@ -2388,6 +2394,38 @@ tracing_poll_pipe(struct file *filp, poll_table *poll_table)
> }
> }
>
> +
> +void default_wait_pipe(struct trace_iterator *iter)
> +{
> + DEFINE_WAIT(wait);
> +
> + prepare_to_wait(&trace_wait, &wait, TASK_INTERRUPTIBLE);
> +
> + if (trace_empty(iter))
> + schedule();
> +
> + finish_wait(&trace_wait, &wait);
> +}
> +
> +/*
> + * This is a make-shift waitqueue.
> + * A tracer might use this callback on some rare cases:
> + *
> + * 1) the current tracer might hold the runqueue lock when it wakes up
> + * a reader, hence a deadlock (sched, function, and function graph tracers)
> + * 2) the function tracers, trace all functions, we don't want
> + * the overhead of calling wake_up and friends
> + * (and tracing them too)
> + *
> + * Anyway, this is really very primitive wakeup.
> + */
> +void poll_wait_pipe(struct trace_iterator *iter)
> +{
> + set_current_state(TASK_INTERRUPTIBLE);
> + /* sleep for 100 msecs, and try again. */
> + schedule_timeout(HZ / 10);
> +}
> +
> /* Must be called with trace_types_lock mutex held. */
> static int tracing_wait_pipe(struct file *filp)
> {
> @@ -2399,30 +2437,14 @@ static int tracing_wait_pipe(struct file *filp)
> return -EAGAIN;
> }
>
> - /*
> - * This is a make-shift waitqueue. The reason we don't use
> - * an actual wait queue is because:
> - * 1) we only ever have one waiter
> - * 2) the tracing, traces all functions, we don't want
> - * the overhead of calling wake_up and friends
> - * (and tracing them too)
> - * Anyway, this is really very primitive wakeup.
> - */
> - set_current_state(TASK_INTERRUPTIBLE);
> - iter->tr->waiter = current;
> -
> mutex_unlock(&trace_types_lock);
>
> - /* sleep for 100 msecs, and try again. */
> - schedule_timeout(HZ/10);
> + iter->trace->wait_pipe(iter);
>
> mutex_lock(&trace_types_lock);
>
> - iter->tr->waiter = NULL;
> -
> - if (signal_pending(current)) {
> + if (signal_pending(current))
> return -EINTR;
> - }
>
> if (iter->trace != current_trace)
> return 0;
> @@ -2438,8 +2460,6 @@ static int tracing_wait_pipe(struct file *filp)
> */
> if (!tracer_enabled && iter->pos)
> break;
> -
> - continue;
> }
>
> return 1;
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index dbff020..eed732c 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -337,18 +337,34 @@ struct tracer_flags {
> #define TRACER_OPT(s, b) .name = #s, .bit = b
>
>
> -/*
> - * A specific tracer, represented by methods that operate on a trace array:
> +/**
> + * struct tracer - a specific tracer and its callbacks to interact with debugfs
> + * @name: the name chosen to select it on the available_tracers file
> + * @init: called when one switches to this tracer (echo name > current_tracer)
> + * @reset: called when one switches to another tracer
> + * @start: called when tracing is unpaused (echo 1 > tracing_enabled)
> + * @stop: called when tracing is paused (echo 0 > tracing_enabled)
> + * @open: called when the trace file is opened
> + * @pipe_open: called when the trace_pipe file is opened
> + * @wait_pipe: override how the user waits for traces on trace_pipe
> + * @close: called when the trace file is released
> + * @read: override the default read callback on trace_pipe
> + * @splice_read: override the default splice_read callback on trace_pipe
> + * @selftest: selftest to run on boot (see trace_selftest.c)
> + * @print_headers: override the first lines that describe your columns
> + * @print_line: callback that prints a trace
> + * @set_flag: signals one of your private flags changed (trace_options file)
> + * @flags: your private flags
> */
> struct tracer {
> const char *name;
> - /* Your tracer should raise a warning if init fails */
> int (*init)(struct trace_array *tr);
> void (*reset)(struct trace_array *tr);
> void (*start)(struct trace_array *tr);
> void (*stop)(struct trace_array *tr);
> void (*open)(struct trace_iterator *iter);
> void (*pipe_open)(struct trace_iterator *iter);
> + void (*wait_pipe)(struct trace_iterator *iter);
> void (*close)(struct trace_iterator *iter);
> ssize_t (*read)(struct trace_iterator *iter,
> struct file *filp, char __user *ubuf,
> @@ -432,6 +448,9 @@ void tracing_generic_entry_update(struct trace_entry *entry,
> unsigned long flags,
> int pc);
>
> +void default_wait_pipe(struct trace_iterator *iter);
> +void poll_wait_pipe(struct trace_iterator *iter);
> +
> void ftrace(struct trace_array *tr,
> struct trace_array_cpu *data,
> unsigned long ip,
> diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
> index 36bf956..e1bceae 100644
> --- a/kernel/trace/trace_functions.c
> +++ b/kernel/trace/trace_functions.c
> @@ -224,6 +224,7 @@ static struct tracer function_trace __read_mostly =
> .init = function_trace_init,
> .reset = function_trace_reset,
> .start = function_trace_start,
> + .wait_pipe = poll_wait_pipe,
> .flags = &func_flags,
> .set_flag = func_set_flag,
> #ifdef CONFIG_FTRACE_SELFTEST
> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index 519a0ca..0ff5cb6 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -757,6 +757,7 @@ static struct tracer graph_trace __read_mostly = {
> .name = "function_graph",
> .open = graph_trace_open,
> .close = graph_trace_close,
> + .wait_pipe = poll_wait_pipe,
> .init = graph_trace_init,
> .reset = graph_trace_reset,
> .print_line = print_graph_function,
> diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
> index 30e14fe..dfe90e4 100644
> --- a/kernel/trace/trace_sched_switch.c
> +++ b/kernel/trace/trace_sched_switch.c
> @@ -221,6 +221,7 @@ static struct tracer sched_switch_trace __read_mostly =
> .reset = sched_switch_trace_reset,
> .start = sched_switch_trace_start,
> .stop = sched_switch_trace_stop,
> + .wait_pipe = poll_wait_pipe,
> #ifdef CONFIG_FTRACE_SELFTEST
> .selftest = trace_selftest_startup_sched_switch,
> #endif
> diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
> index 96d7164..8c2d1c3 100644
> --- a/kernel/trace/trace_sched_wakeup.c
> +++ b/kernel/trace/trace_sched_wakeup.c
> @@ -380,6 +380,7 @@ static struct tracer wakeup_rt_tracer __read_mostly =
> .reset = wakeup_tracer_reset,
> .start = wakeup_tracer_start,
> .stop = wakeup_tracer_stop,
> + .wait_pipe = poll_wait_pipe,
> .print_max = 1,
> #ifdef CONFIG_FTRACE_SELFTEST
> .selftest = trace_selftest_startup_wakeup,
> --
> 1.6.1
>
>
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