linux-kernel - Re: [RFC PATCH 82/86] treewide: mtd: remove cond

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Date:   Wed, 8 Nov 2023 12:21:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, David.Laight@...lab.com,
        richard@....at, mjguzik@...il.com,
        Vignesh Raghavendra <vigneshr@...com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tudor Ambarus <tudor.ambarus@...aro.org>,
        Pratyush Yadav <pratyush@...nel.org>
Subject: Re: [RFC PATCH 82/86] treewide: mtd: remove cond_resched()

On Wed, 8 Nov 2023 16:32:36 +0000
Matthew Wilcox <willy@...radead.org> wrote:

> On Wed, Nov 08, 2023 at 05:28:27PM +0100, Miquel Raynal wrote:
> > > --- a/drivers/mtd/nand/raw/nand_legacy.c
> > > +++ b/drivers/mtd/nand/raw/nand_legacy.c
> > > @@ -203,7 +203,13 @@ void nand_wait_ready(struct nand_chip *chip)
> > >  	do {
> > >  		if (chip->legacy.dev_ready(chip))
> > >  			return;
> > > -		cond_resched();
> > > +		/*
> > > +		 * Use a cond_resched_stall() to avoid spinning in
> > > +		 * a tight loop.
> > > +		 * Though, given that the timeout is in milliseconds,
> > > +		 * maybe this should timeout or event wait?  
> > 
> > Event waiting is precisely what we do here, with the hardware access
> > which are available in this case. So I believe this part of the comment
> > (in general) is not relevant. Now regarding the timeout I believe it is
> > closer to the second than the millisecond, so timeout-ing is not
> > relevant either in most cases (talking about mtd/ in general).  
> 
> I think you've misunderstood what Ankur wrote here.  What you're
> currently doing is spinning in a very tight loop.  The comment is
> suggesting you might want to msleep(1) or something to avoid burning CPU
> cycles.  It'd be even better if the hardware could signal you somehow,
> but I bet it can't.
> 

Oh how I wish we could bring back the old PREEMPT_RT cpu_chill()...

#define cpu_chill()	msleep(1)

;-)

-- Steve


> > > +		 */
> > > +		cond_resched_stall();
> > >  	} while (time_before(jiffies, timeo));  
> > 
> > Thanks,
> > Miquèl
> >