linux-kernel - Re: [PATCH 4.19 01/71] drm/i915: Break up error capture compression loops with cond

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Date:   Mon, 9 Nov 2020 19:20:46 +0100
From:   Pavel Machek <pavel@....cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Mika Kuoppala <mika.kuoppala@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: [PATCH 4.19 01/71] drm/i915: Break up error capture compression
 loops with cond_resched()

Hi!

> commit 7d5553147613b50149238ac1385c60e5c7cacb34 upstream.
> 
> As the error capture will compress user buffers as directed to by the
> user, it can take an arbitrary amount of time and space. Break up the
> compression loops with a call to cond_resched(), that will allow other
> processes to schedule (avoiding the soft lockups) and also serve as a
> warning should we try to make this loop atomic in the future.

This was queued for 4.19-stable, but is very likely wrong.

> Testcase: igt/gem_exec_capture/many-*
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@...ux.intel.com>
> Cc: stable@...r.kernel.org
> Reviewed-by: Mika Kuoppala <mika.kuoppala@...ux.intel.com>
> Link: https://patchwork.freedesktop.org/patch/msgid/20200916090059.3189-2-chris@chris-wilson.co.uk
> (cherry picked from commit 293f43c80c0027ff9299036c24218ac705ce584e)
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@...el.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

> @@ -347,6 +349,7 @@ static int compress_page(struct compress
>  	if (!i915_memcpy_from_wc(ptr, src, PAGE_SIZE))
>  		memcpy(ptr, src, PAGE_SIZE);
>  	dst->pages[dst->page_count++] = ptr;
> +	cond_resched();
>  
>  	return 0;
>  }

4.19 compress_page begins with

static int compress_page(struct compress *c,
...
        page = __get_free_page(GFP_ATOMIC | __GFP_NOWARN);

and likely may not sleep. That changed with commit
a42f45a2a85998453078, but that one is not present in 4.19..

I believe we don't need this in stable: dumping of error file will not
take so long to trigger softlockup detectors... 

Best regards,
                                                                Pavel
								
> 

-- 
http://www.livejournal.com/~pavelmachek

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