linux-kernel - Re: [PATCH V2 18/19] clocksource: add C-SKY clocksource drivers

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Date:   Wed, 4 Jul 2018 16:35:43 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Guo Ren <ren_guo@...ky.com>
cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        daniel.lezcano@...aro.org, jason@...edaemon.net, arnd@...db.de,
        c-sky_gcc_upstream@...ky.com, gnu-csky@...tor.com,
        thomas.petazzoni@...tlin.com, wbx@...ibc-ng.org, green.hu@...il.com
Subject: Re: [PATCH V2 18/19] clocksource: add C-SKY clocksource drivers

On Wed, 4 Jul 2018, Guo Ren wrote:
> On Tue, Jul 03, 2018 at 11:39:05AM +0200, Thomas Gleixner wrote:
> > > +static inline u64 get_ccvr(void)
> > > +{
> > > +	u32 lo, hi, t;
> > > +
> > > +	do {
> > > +		hi = mfcr(PTIM_CCVR_HI);
> > > +		lo = mfcr(PTIM_CCVR_LO);
> > > +		t  = mfcr(PTIM_CCVR_HI);
> > > +	} while(t != hi);
> > 
> > No idea which frequency this timer ticks at, but if the 32 bit wrap does
> > not come too fast, then you really should avoid that loop. That function is
> > called very frequently.
> 
> 0000006c <clksrc_read>:
> 		hi = mfcr(PTIM_CCVR_HI);
>   6c:	c1c26023 	mfcr      	r3, cr<2, 14>
> 		lo = mfcr(PTIM_CCVR_LO);
>   70:	c1c36021 	mfcr      	r1, cr<3, 14>
> 		t  = mfcr(PTIM_CCVR_HI);
>   74:	c1c26022 	mfcr      	r2, cr<2, 14>
> 	} while(t != hi);
>   78:	648e      	cmpne      	r3, r2
>   7a:	0bf9      	bt      	0x6c	// 6c <clksrc_read>
> 
> When two read cr<2, 14> is not equal, we'll retry. So only when
> CCVR_LO is at 0xffffffff between the two read of CCVR_HI. That's very
> very small probability event for "bt 0x6c".
> 
> Don't worry about the "do {...} whie(t != hi)", it's no performance issue.

But _three_ mfcr plus a conditional jump which _cannot_ be predicted are a
performance issue. When you can replace that with a single mfcr, then you
win a lot, really. The time keeping and the sched clock code can handle
that nicely unless you really have fast wrap arounds on the LO word.

Thanks,

	tglx