From: Steven Rostedt (Red Hat) Date: Wed, 11 Feb 2015 03:14:53 +0000 (-0500) Subject: ring-buffer: Do not wake up a splice waiter when page is not full X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=1e0d6714aceb770b04161fbedd7765d0e1fc27bd;p=GitHub%2FLineageOS%2FG12%2Fandroid_kernel_amlogic_linux-4.9.git ring-buffer: Do not wake up a splice waiter when page is not full When an application connects to the ring buffer via splice, it can only read full pages. Splice does not work with partial pages. If there is not enough data to fill a page, the splice command will either block or return -EAGAIN (if set to nonblock). Code was added where if the page is not full, to just sleep again. The problem is, it will get woken up again on the next event. That is, when something is written into the ring buffer, if there is a waiter it will wake it up. The waiter would then check the buffer, see that it still does not have enough data to fill a page and go back to sleep. To make matters worse, when the waiter goes back to sleep, it could cause another event, which would wake it back up again to see it doesn't have enough data and sleep again. This produces a tremendous overhead and fills the ring buffer with noise. For example, recording sched_switch on an idle system for 10 seconds produces 25,350,475 events!!! Create another wait queue for those waiters wanting full pages. When an event is written, it only wakes up waiters if there's a full page of data. It does not wake up the waiter if the page is not yet full. After this change, recording sched_switch on an idle system for 10 seconds produces only 800 events. Getting rid of 25,349,675 useless events (99.9969% of events!!), is something to take seriously. Cc: stable@vger.kernel.org # 3.16+ Cc: Rabin Vincent Fixes: e30f53aad220 "tracing: Do not busy wait in buffer splice" Signed-off-by: Steven Rostedt --- diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 96079180de3d..5040d44fe5a3 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -445,7 +445,10 @@ int ring_buffer_print_page_header(struct trace_seq *s) struct rb_irq_work { struct irq_work work; wait_queue_head_t waiters; + wait_queue_head_t full_waiters; bool waiters_pending; + bool full_waiters_pending; + bool wakeup_full; }; /* @@ -527,6 +530,10 @@ static void rb_wake_up_waiters(struct irq_work *work) struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work); wake_up_all(&rbwork->waiters); + if (rbwork->wakeup_full) { + rbwork->wakeup_full = false; + wake_up_all(&rbwork->full_waiters); + } } /** @@ -551,9 +558,11 @@ int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full) * data in any cpu buffer, or a specific buffer, put the * caller on the appropriate wait queue. */ - if (cpu == RING_BUFFER_ALL_CPUS) + if (cpu == RING_BUFFER_ALL_CPUS) { work = &buffer->irq_work; - else { + /* Full only makes sense on per cpu reads */ + full = false; + } else { if (!cpumask_test_cpu(cpu, buffer->cpumask)) return -ENODEV; cpu_buffer = buffer->buffers[cpu]; @@ -562,7 +571,10 @@ int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full) while (true) { - prepare_to_wait(&work->waiters, &wait, TASK_INTERRUPTIBLE); + if (full) + prepare_to_wait(&work->full_waiters, &wait, TASK_INTERRUPTIBLE); + else + prepare_to_wait(&work->waiters, &wait, TASK_INTERRUPTIBLE); /* * The events can happen in critical sections where @@ -584,7 +596,10 @@ int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full) * that is necessary is that the wake up happens after * a task has been queued. It's OK for spurious wake ups. */ - work->waiters_pending = true; + if (full) + work->full_waiters_pending = true; + else + work->waiters_pending = true; if (signal_pending(current)) { ret = -EINTR; @@ -613,7 +628,10 @@ int ring_buffer_wait(struct ring_buffer *buffer, int cpu, bool full) schedule(); } - finish_wait(&work->waiters, &wait); + if (full) + finish_wait(&work->full_waiters, &wait); + else + finish_wait(&work->waiters, &wait); return ret; } @@ -1228,6 +1246,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int nr_pages, int cpu) init_completion(&cpu_buffer->update_done); init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters); init_waitqueue_head(&cpu_buffer->irq_work.waiters); + init_waitqueue_head(&cpu_buffer->irq_work.full_waiters); bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()), GFP_KERNEL, cpu_to_node(cpu)); @@ -2799,6 +2818,8 @@ static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer, static __always_inline void rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) { + bool pagebusy; + if (buffer->irq_work.waiters_pending) { buffer->irq_work.waiters_pending = false; /* irq_work_queue() supplies it's own memory barriers */ @@ -2810,6 +2831,15 @@ rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) /* irq_work_queue() supplies it's own memory barriers */ irq_work_queue(&cpu_buffer->irq_work.work); } + + pagebusy = cpu_buffer->reader_page == cpu_buffer->commit_page; + + if (!pagebusy && cpu_buffer->irq_work.full_waiters_pending) { + cpu_buffer->irq_work.wakeup_full = true; + cpu_buffer->irq_work.full_waiters_pending = false; + /* irq_work_queue() supplies it's own memory barriers */ + irq_work_queue(&cpu_buffer->irq_work.work); + } } /**