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SubjectRe: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!
> Well, actually it's been optimized a lot from the first instances.
> Note, tracepoints need to be quite generic, so where it can be optimized
> is not obvious.

There is quite a bit of stuff that doesn't change from trace point to
trace point. So you could just cache all these decisions into a single
per cpu variable, and invalidate if anything changes the conditions.

> >
> > Functions with # of instructions:
> >
> > 25 trace_event_raw_event_sched_switch
>
> This is from the TRACE_EVENT macro.
>
> if (trace_trigger_soft_disabled(trace_file)) \
> return; \

Cache?

> if ((trace_file->flags & EVENT_FILE_FL_PID_FILTER) &&
> trace_event_ignore_this_pid(trace_file))
> return NULL;
>
> *current_rb = trace_file->tr->trace_buffer.buffer;

Indirection could be cached.

>
> if ((trace_file->flags &
> (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
> (entry = this_cpu_read(trace_buffered_event))) {

Multiple tests of trace_file->flags in different functions,
could be all folded into a single test.

>
>
> Note: Some of these if statements can be encapsulated with jump labels, as they
> are false pretty much all of the time.
>
>
> /* Try to use the per cpu buffer first */
> val = this_cpu_inc_return(trace_buffered_event_cnt);
> if (val == 1) {
> trace_event_setup(entry, type, flags, pc);
> entry->array[0] = len;
> return entry;
> }
> this_cpu_dec(trace_buffered_event_cnt);

This can be pre cached in a fast path. If true just point per cpu
variable directly to buffer and invalidate if buffer changes or is full.

> preempt_disable_notrace();
>
> if (unlikely(atomic_read(&buffer->record_disabled)))
> goto out;

Cache.
>
> cpu = raw_smp_processor_id();
>
> if (unlikely(!cpumask_test_cpu(cpu, buffer->cpumask)))
> goto out;

Not needed for per cpu buffers in a fast path.

>
> cpu_buffer = buffer->buffers[cpu];
>
> if (unlikely(atomic_read(&cpu_buffer->record_disabled)))
> goto out;
cache

>
> if (unlikely(length > BUF_MAX_DATA_SIZE))
> goto out;
>
> if (unlikely(trace_recursive_lock(cpu_buffer)))
> goto out;

locking should invalidate state on this cpu, so shouldn't be needed

> u64 diff;
>
> rb_start_commit(cpu_buffer);
>
> #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
> /*
> * Due to the ability to swap a cpu buffer from a buffer
> * it is possible it was swapped before we committed.
> * (committing stops a swap). We check for it here and
> * if it happened, we have to fail the write.
> */
> barrier();
> if (unlikely(ACCESS_ONCE(cpu_buffer->buffer) != buffer)) {
> local_dec(&cpu_buffer->committing);
> local_dec(&cpu_buffer->commits);
> return NULL;
> }

swapping should invalidate state centrally (and then run slow path like
current path)

> /*
> * We allow for interrupts to reenter here and do a trace.
> * If one does, it will cause this original code to loop
> * back here. Even with heavy interrupts happening, this
> * should only happen a few times in a row. If this happens
> * 1000 times in a row, there must be either an interrupt
> * storm or we have something buggy.
> * Bail!
> */
> if (RB_WARN_ON(cpu_buffer, ++nr_loops > 1000))
> goto out_fail;

didn't you disable interrupts earlier?

> /* Did the write stamp get updated already? */
> if (likely(info.ts >= cpu_buffer->write_stamp)) {
> info.delta = diff;
> if (unlikely(test_time_stamp(info.delta)))
> rb_handle_timestamp(cpu_buffer, &info);
> }

Not sure why a write stamp is needed. isn't that the last few entries?

...

-Andi

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