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

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Date:	5 Jun 2015 04:24:53 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, mingo@...nel.org
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()

> Ingo Molnar wrote:
>* George Spelvin <linux@...izon.com> wrote:
>> Did you use rtc_cmos_read()?  [...]

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

Apologies for doubting you!

>> /* This is skanky stuff that requries rewritten RTC locking to do properly */

> [ Note that no RTC locking is needed so early during bootup: this is
>   the boot CPU only, with only a single task running, guaranteed. ]

Yes, I guessed I could get away with it, but I hadn't traced the code
far enough to be sure.  But obviously I should either completely omit the
locking, or do it right.  Half-assed is all-wrong.

> note the 'loops' column. When it's around 117, then the read cost corresponds 
> roughly to the cheap-ish INB cost you have measured: 4188 cycles/loop.
> 
> But note the frequent 30-40k cycles/loop outliers. They dominate the measurement 
> so filtering might not help.

I don't quite understand hoe the numbers are derived.  Why does 200K
cycles/loop give 13 loops, while 35K cycles/loop gives 7?  Is cycles/loop
a maximum?

> And this is on a 'boring' 10 years old PC (Nvidia CK804 southbridge), with no HPET 
> and nothing particularly fancy that I'm aware of. I tried this system first 
> because I expected it to work and expected problems (with RTCs being emulated via 
> the HPET) on more modern systems.
> 
> If the RTC polling method is not reliable here, it might be doubly problematic on 
> other systems.

This is definitely an "if it ain't broke, don't fix it" area.
Trying things is interesting; actually changing the kernel is not
to be undertaken lightly.
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