linux-kernel - Re: [PATCH] aoe: Use 64-bit timestamp in frame

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Date:	Mon, 11 May 2015 11:59:04 -0400
From:	Ed Cashin <ed.cashin@....org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org, y2038@...ts.linaro.org,
	Tina Ruchandani <ruchandani.tina@...il.com>
Subject: Re: [PATCH] aoe: Use 64-bit timestamp in frame

I would like to see some performance measurements for this patch on a system with fast storage and multiple 10 GbE links. 

If not, at least a good analysis of the expected performance impact the patch will have on major architectures. 

Tonight I will think about whether the 2038 thing even matters or whether we just need a comment explaining why it's safe.

On May 11, 2015 11:38 AM, Arnd Bergmann <arnd@...db.de> wrote:
>
> On Monday 11 May 2015 08:05:05 Tina Ruchandani wrote: 
> > 'struct frame' uses two variables to store the sent timestamp - 'struct 
> > timeval' and jiffies. jiffies is used to avoid discrepancies caused by 
> > updates to system time. 'struct timeval' uses 32-bit representation for 
> > seconds which will overflow in year 2038. 
> > This patch does the following: 
> > - Replace the use of 'struct timeval' and jiffies with ktime_t, which 
> > is a 64-bit timestamp and is year 2038 safe. 
> > - ktime_t provides both long range (like jiffies) and high resolution 
> > (like timeval). Using ktime_get (monotonic time) instead of wall-clock 
> > time prevents any discprepancies caused by updates to system time. 
> > 
> > Signed-off-by: Tina Ruchandani <ruchandani.tina@...il.com> 
>
> Very nice! 
>
> > @@ -499,32 +497,15 @@ resend(struct aoedev *d, struct frame *f) 
> >  static int 
> >  tsince_hr(struct frame *f) 
> >  { 
> > - struct timeval now; 
> > + ktime_t now; 
> >  int n; 
> >  
> > - do_gettimeofday(&now); 
> > - n = now.tv_usec - f->sent.tv_usec; 
> > - n += (now.tv_sec - f->sent.tv_sec) * USEC_PER_SEC; 
> > + now = ktime_get(); 
> > + n = ktime_to_us(ktime_sub(now, f->sent)); 
> >  
>
> I would cut four extra lines by writing this as 
>
> return ktime_us_delta(ktime_get(), f->sent)); 
>
> but the effect is exactly the same. 
>
> With that change, please add 
>
> Reviewed-by: Arnd Bergmann <arnd@...db.de> 
>
> Arnd