linux-kernel - Re: [PATCH RFC 01/14] block, bfq: introduce the BFQ-v0 I/O scheduler as an extra scheduler

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Date:   Sat, 18 Mar 2017 15:24:34 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "paolo.valente@...aro.org" <paolo.valente@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "fchecconi@...il.com" <fchecconi@...il.com>,
        "linus.walleij@...aro.org" <linus.walleij@...aro.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "avanzini.arianna@...il.com" <avanzini.arianna@...il.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "tj@...nel.org" <tj@...nel.org>,
        "ulf.hansson@...aro.org" <ulf.hansson@...aro.org>
Subject: Re: [PATCH RFC 01/14] block, bfq: introduce the BFQ-v0 I/O
 scheduler as an extra scheduler

On Sat, 2017-03-18 at 08:08 -0400, Paolo Valente wrote:
> > Il giorno 06 mar 2017, alle ore 14:40, Bart Van Assche <bart.vanassche@...disk.com> ha scritto:
> > > +#define BFQ_BFQQ_FNS(name)						\
> > > +static void bfq_mark_bfqq_##name(struct bfq_queue *bfqq)		\
> > > +{									\
> > > +	(bfqq)->flags |= (1 << BFQ_BFQQ_FLAG_##name);			\
> > > +}									\
> > > +static void bfq_clear_bfqq_##name(struct bfq_queue *bfqq)		\
> > > +{									\
> > > +	(bfqq)->flags &= ~(1 << BFQ_BFQQ_FLAG_##name);			\
> > > +}									\
> > > +static int bfq_bfqq_##name(const struct bfq_queue *bfqq)		\
> > > +{									\
> > > +	return ((bfqq)->flags & (1 << BFQ_BFQQ_FLAG_##name)) != 0;	\
> > > +}
> > 
> > Are the bodies of the above functions duplicates of __set_bit(),
> > __clear_bit() and test_bit()?
> 
> Yes.  We wrapped them into functions, because writing mark_flag_name
> seemed more readable than writing the implementation of the marking of the
> flag.

Please do not open-code __set_bit(), __clear_bit() and test_bit() but use
these macros instead.

> > > +	} else
> > > +		/*
> > > +		 * Async queues get always the maximum possible
> > > +		 * budget, as for them we do not care about latency
> > > +		 * (in addition, their ability to dispatch is limited
> > > +		 * by the charging factor).
> > > +		 */
> > > +		budget = bfqd->bfq_max_budget;
> > > +
> > 
> > Please balance braces. Checkpatch should have warned about the use of "}
> > else" instead of "} else {".
> 
> No warning, I guess because the body of the else contains only a
> simple instruction.  Just to learn for the future: what's the
> rationale for adding braces here, but not imposing braces everywhere
> for single-instruction bodies?

It's a general style recommendation for all kernel code: if braces are used
for one side of an if-statement, also use braces for the other side, and
definitely if that other side consists of multiple lines due to a comment.

Bart.