linux-kernel - Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit

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Date:	Fri, 5 Jun 2015 07:58:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	George Spelvin <linux@...izon.com>
Cc:	adrian.hunter@...el.com, ak@...ux.intel.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, luto@...capital.net,
	tglx@...utronix.de, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in
 quick_pit_calibrate()


* George Spelvin <linux@...izon.com> wrote:

> Ingo Molnar wrote:
> > - Alternatively, I also tried a different method: to set up the RTC
> >   periodic IRQ during early boot, but not have an IRQ handler, polling
> >   RTC_PF in the rtc_cmos_read(RTC_INTR_FLAGS) IRQ status byte.
> >
> >   Unfortunately when I do this then PIO based RTC accesses can take
> >   tens of thousands of cycles, and the resulting jitter is pretty bad
> >   and hard to filter:
> 
> Did you use rtc_cmos_read()?  [...]

Yeah, so initially I did, but then after I noticed the overhead I introduced:

+unsigned char rtc_cmos_read_again(void)
+{
+       return inb(RTC_PORT(1));
+}
+

which compiles to a single INB instruction.

This didn't change the delay/cost behavior.

The numbers I cited, with tens of thousands of cycles per iteration, were from 
such an optimized poll loop already.

Thanks,

	Ingo
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