Messages in this thread | | | Date | Fri, 26 Jun 2020 07:42:10 +1000 | From | Anton Blanchard <> | Subject | Re: [PATCH] ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU |
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Hi Nick,
> On a 144 thread system, `perf ftrace` takes about 20 seconds to start > up, due to calling synchronize_rcu() for each CPU. > > cat /proc/108560/stack > 0xc0003e7eb336f470 > __switch_to+0x2e0/0x480 > __wait_rcu_gp+0x20c/0x220 > synchronize_rcu+0x9c/0xc0 > ring_buffer_reset_cpu+0x88/0x2e0 > tracing_reset_online_cpus+0x84/0xe0 > tracing_open+0x1d4/0x1f0 > > On a system with 10x more threads, it starts to become an annoyance. > > Batch these up so we disable all the per-cpu buffers first, then > synchronize_rcu() once, then reset each of the buffers. This brings > the time down to about 0.5s.
It's gone from somewhere more than 10 minutes (I gave up waiting) to 3 seconds. Nice work!
Tested-by: Anton Blanchard <anton@ozlabs.org>
Thanks, Anton
> Cc: Paul McKenney <paulmck@kernel.org> > Cc: Anton Blanchard <anton@ozlabs.org> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Nicholas Piggin <npiggin@gmail.com> > --- > include/linux/ring_buffer.h | 1 + > kernel/trace/ring_buffer.c | 85 > +++++++++++++++++++++++++++++++------ kernel/trace/trace.c | > 4 +- 3 files changed, 73 insertions(+), 17 deletions(-) > > diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h > index c76b2f3b3ac4..136ea0997e6d 100644 > --- a/include/linux/ring_buffer.h > +++ b/include/linux/ring_buffer.h > @@ -143,6 +143,7 @@ bool ring_buffer_iter_dropped(struct > ring_buffer_iter *iter); unsigned long ring_buffer_size(struct > trace_buffer *buffer, int cpu); > void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu); > +void ring_buffer_reset_online_cpus(struct trace_buffer *buffer); > void ring_buffer_reset(struct trace_buffer *buffer); > > #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index b8e1ca48be50..3f1fd02bd14a 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -270,6 +270,9 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data); > #define for_each_buffer_cpu(buffer, cpu) \ > for_each_cpu(cpu, buffer->cpumask) > > +#define for_each_online_buffer_cpu(buffer, cpu) \ > + for_each_cpu_and(cpu, buffer->cpumask, cpu_online_mask) > + > #define TS_SHIFT 27 > #define TS_MASK ((1ULL << TS_SHIFT) - 1) > #define TS_DELTA_TEST (~TS_MASK) > @@ -4484,6 +4487,26 @@ rb_reset_cpu(struct ring_buffer_per_cpu > *cpu_buffer) rb_head_page_activate(cpu_buffer); > } > > +/* Must have disabled the cpu buffer then done a synchronize_rcu */ > +static void reset_disabled_cpu_buffer(struct ring_buffer_per_cpu > *cpu_buffer) +{ > + unsigned long flags; > + > + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); > + > + if (RB_WARN_ON(cpu_buffer, > local_read(&cpu_buffer->committing))) > + goto out; > + > + arch_spin_lock(&cpu_buffer->lock); > + > + rb_reset_cpu(cpu_buffer); > + > + arch_spin_unlock(&cpu_buffer->lock); > + > + out: > + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); > +} > + > /** > * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer > * @buffer: The ring buffer to reset a per cpu buffer of > @@ -4492,7 +4515,6 @@ rb_reset_cpu(struct ring_buffer_per_cpu > *cpu_buffer) void ring_buffer_reset_cpu(struct trace_buffer *buffer, > int cpu) { > struct ring_buffer_per_cpu *cpu_buffer = > buffer->buffers[cpu]; > - unsigned long flags; > > if (!cpumask_test_cpu(cpu, buffer->cpumask)) > return; > @@ -4503,24 +4525,42 @@ void ring_buffer_reset_cpu(struct > trace_buffer *buffer, int cpu) /* Make sure all commits have finished > */ synchronize_rcu(); > > - raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); > + reset_disabled_cpu_buffer(cpu_buffer); > > - if (RB_WARN_ON(cpu_buffer, > local_read(&cpu_buffer->committing))) > - goto out; > + atomic_dec(&cpu_buffer->record_disabled); > + atomic_dec(&cpu_buffer->resize_disabled); > +} > +EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); > > - arch_spin_lock(&cpu_buffer->lock); > +/** > + * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer > + * @buffer: The ring buffer to reset a per cpu buffer of > + * @cpu: The CPU buffer to be reset > + */ > +void ring_buffer_reset_online_cpus(struct trace_buffer *buffer) > +{ > + struct ring_buffer_per_cpu *cpu_buffer; > + int cpu; > > - rb_reset_cpu(cpu_buffer); > + for_each_online_buffer_cpu(buffer, cpu) { > + cpu_buffer = buffer->buffers[cpu]; > > - arch_spin_unlock(&cpu_buffer->lock); > + atomic_inc(&cpu_buffer->resize_disabled); > + atomic_inc(&cpu_buffer->record_disabled); > + } > > - out: > - raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); > + /* Make sure all commits have finished */ > + synchronize_rcu(); > > - atomic_dec(&cpu_buffer->record_disabled); > - atomic_dec(&cpu_buffer->resize_disabled); > + for_each_online_buffer_cpu(buffer, cpu) { > + cpu_buffer = buffer->buffers[cpu]; > + > + reset_disabled_cpu_buffer(cpu_buffer); > + > + atomic_dec(&cpu_buffer->record_disabled); > + atomic_dec(&cpu_buffer->resize_disabled); > + } > } > -EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); > > /** > * ring_buffer_reset - reset a ring buffer > @@ -4528,10 +4568,27 @@ EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu); > */ > void ring_buffer_reset(struct trace_buffer *buffer) > { > + struct ring_buffer_per_cpu *cpu_buffer; > int cpu; > > - for_each_buffer_cpu(buffer, cpu) > - ring_buffer_reset_cpu(buffer, cpu); > + for_each_buffer_cpu(buffer, cpu) { > + cpu_buffer = buffer->buffers[cpu]; > + > + atomic_inc(&cpu_buffer->resize_disabled); > + atomic_inc(&cpu_buffer->record_disabled); > + } > + > + /* Make sure all commits have finished */ > + synchronize_rcu(); > + > + for_each_buffer_cpu(buffer, cpu) { > + cpu_buffer = buffer->buffers[cpu]; > + > + reset_disabled_cpu_buffer(cpu_buffer); > + > + atomic_dec(&cpu_buffer->record_disabled); > + atomic_dec(&cpu_buffer->resize_disabled); > + } > } > EXPORT_SYMBOL_GPL(ring_buffer_reset); > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index ec44b0e2a19c..9a26a1c875ae 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -2003,7 +2003,6 @@ static void tracing_reset_cpu(struct > array_buffer *buf, int cpu) void tracing_reset_online_cpus(struct > array_buffer *buf) { > struct trace_buffer *buffer = buf->buffer; > - int cpu; > > if (!buffer) > return; > @@ -2015,8 +2014,7 @@ void tracing_reset_online_cpus(struct > array_buffer *buf) > buf->time_start = buffer_ftrace_now(buf, buf->cpu); > > - for_each_online_cpu(cpu) > - ring_buffer_reset_cpu(buffer, cpu); > + ring_buffer_reset_online_cpus(buffer); > > ring_buffer_record_enable(buffer); > }
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