Re: [PATCH 6/6] writeback: remove unneeded GDTC_INIT_NO_WB

From: Jan Kara <jack@suse.cz>
To: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Tejun Heo <tj@kernel.org>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	willy@infradead.org, bfoster@redhat.com, jack@suse.cz,
	dsterba@suse.com, mjguzik@gmail.com, dhowells@redhat.com,
	peterz@infradead.org
Subject: Re: [PATCH 6/6] writeback: remove unneeded GDTC_INIT_NO_WB
Date: Wed, 27 Mar 2024 10:33:09 +0100	[thread overview]
Message-ID: <20240327093309.ejuzjus2zcixb4qt@quack3> (raw)
In-Reply-To: <becdb16b-a318-ec05-61d2-d190541ae997@huaweicloud.com>

On Thu 21-03-24 15:12:21, Kemeng Shi wrote:
> 
> 
> on 3/20/2024 11:15 PM, Tejun Heo wrote:
> > Hello,
> > 
> > On Wed, Mar 20, 2024 at 07:02:22PM +0800, Kemeng Shi wrote:
> >> We never use gdtc->dom set with GDTC_INIT_NO_WB, just remove unneeded
> >> GDTC_INIT_NO_WB
> >>
> >> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> > ...
> >>  void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
> >>  {
> >> -	struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
> >> +	struct dirty_throttle_control gdtc = { };
> > 
> > Even if it's currently not referenced, wouldn't it still be better to always
> > guarantee that a dtc's dom is always initialized? I'm not sure what we get
> > by removing this.
> As we explicitly use GDTC_INIT_NO_WB to set global_wb_domain before
> calculating dirty limit with domain_dirty_limits, I intuitively think the
> dirty limit calculation in domain_dirty_limits is related to
> global_wb_domain when CONFIG_WRITEBACK_CGROUP is enabled while the truth
> is not. So this is a little confusing to me.

I'm not sure I understand your confusion. domain_dirty_limits() calculates
the dirty limit (and background dirty limit) for the dirty_throttle_control
passed in. If you pass dtc initialized with GDTC_INIT[_NO_WB], it will
compute global dirty limits. If the dtc passed in is initialized with
MDTC_INIT() it will compute cgroup specific dirty limits.

Now because domain_dirty_limits() does not scale the limits based on each
device throughput - that is done only later in __wb_calc_thresh() to avoid
relatively expensive computations when we don't need them - and also
because the effective dirty limit (dtc->dom->dirty_limit) is not updated by
domain_dirty_limits(), domain_dirty_limits() does not need dtc->dom at all.
But that is a technical detail of implementation and I don't want this
technical detail to be relied on by even more code.

What might have confused you is that GDTC_INIT_NO_WB is defined to be empty
when CONFIG_CGROUP_WRITEBACK is disabled. But this is only because in that
case dtc_dom() function unconditionally returns global_wb_domain so we
don't bother with initializing (or even having) the 'dom' field anywhere.

Now I agree this whole code is substantially confusing and complex and it
would all deserve some serious thought how to make it more readable. But
even after thinking about it again I don't think removing GDTC_INIT_NO_WB is
the right way to go.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR