linux-kernel - drm_cflush_sg() loops for over 3ms

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Date:   Mon, 13 Jan 2020 14:34:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'maarten.lankhorst@...ux.intel.com'" 
        <maarten.lankhorst@...ux.intel.com>,
        "'mripard@...nel.org'" <mripard@...nel.org>,
        "'sean@...rly.run'" <sean@...rly.run>,
        "'airlied@...ux.ie'" <airlied@...ux.ie>,
        "'daniel@...ll.ch'" <daniel@...ll.ch>,
        "'dri-devel@...ts.freedesktop.org'" <dri-devel@...ts.freedesktop.org>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: drm_cflush_sg() loops for over 3ms

I've been looking at why some RT processes don't get scheduled promptly.
In my test the RT process's affinity ties it to a single cpu (this may not be such
a good idea as it seems).

What I've found is that the Intel i915 graphics driver uses the 'events_unbound'
kernel worker thread to periodically execute drm_cflush_sg().
(see https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/drm_cache.c)

I'm guessing this is to ensure that any writes to graphics memory become
visible is a semi-timely manner.

This loop takes about 1us per iteration split fairly evenly between whatever is in
for_each_sg_page() and drm_cflush_page().
With a 2560x1440 display the loop count is 3600 (4 bytes/pixel) and the whole
function takes around 3.3ms.

Since the kernel isn't pre-emptive (I though that wasn't much harder than SMP)
nothing else can run on that cpu until the loop finishes.

Adding a cond_resched() to the loop (maybe every 64 iterations) will
allow higher priority processes to run.
But really the code needs to be a lot faster.

I actually suspect that the (I assume IPI based) wbinv_on_all_cpus() would be
a lot faster - especially done by a per-cpu work queue?

I had moderate difficulty getting from the process (kworker/u8:3) to the
name of the worker thread pool, never mind the actual work.
Fortunately it runs so long that some of the output from 'echo t >/proc/sysrq-trigger'
still linked the pid (which I knew from ftrace scheduler events (and schedviz))
to the actual work item name.
(Oh, after I'd written a program to tidy up the raw ftrace output so schedviz
didn't barf on a trace that had wrapped.)

Is there anything in /proc (etc) that shows all the work queues and their current
work?

	David

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