tracing: Fix poll() and select() do not work on per_cpu trace_pipe and trace_pipe_raw

poll() and select() on per_cpu trace_pipe and trace_pipe_raw do not work
since kernel 6.1-rc6. This issue is seen after the commit
42fb0a1 ("tracing/ring-buffer: Have
polling block on watermark").

This issue is firstly detected and reported, when testing the CXL error
events in the rasdaemon and also erified using the test application for poll()
and select().

This issue occurs for the per_cpu case, when calling the ring_buffer_poll_wait(),
in kernel/trace/ring_buffer.c, with the buffer_percent > 0 and then wait until the
percentage of pages are available. The default value set for the buffer_percent is 50
in the kernel/trace/trace.c.

As a fix, allow userspace application could set buffer_percent as 0 through
the buffer_percent_fops, so that the task will wake up as soon as data is added
to any of the specific cpu buffer.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Cc: [email protected]
Fixes: 42fb0a1 ("tracing/ring-buffer: Have polling block on watermark")
Signed-off-by: Shiju Jose <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: shijujose4

kernel/trace/trace.c

@@ -9148,9 +9148,6 @@ buffer_percent_write(struct file *filp, const char __user *ubuf,
 	if (val > 100)
 		return -EINVAL;
 
-	if (!val)
-		val = 1;
-
 	tr->buffer_percent = val;
 
 	(*ppos)++;