Message-ID: <AANLkTikTjBAZGBF2BirRpayiL6PX=ewmh+ZEnZkZBr=s@mail.gmail.com>
Date:	Thu, 10 Mar 2011 17:03:51 +1100
From:	Andrew Worsley <amworsley@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Fix for critical bogoMIPS intermittent calculation failure

Please find attach a fix to the TSC  (Time Stamp Counter) based
bogoMIPS calculation used on secondary CPUs which has two faults:

   1.  Not handling wrapping of the lower 32 bits of the TSC counter
on 32bit kernel - perhaps TSC is not reset by a warm reset?
   2.  TSC and Jiffies are no incrementing together properly. Either
jiffies increment too quickly or Time Stamp Counter isn't incremented
in during an SMI but the real time clock is and jiffies are incremented.

Case 1 can result in a factor of 16 too large a value which makes
udelay() values too small and can cause mysterious driver errors.
Case 2 appears to give smaller 10-15% errors after averaging but
enough to cause occasional failures on my own board

I have tested this code on my own branch and attach patch suitable for
current kernel code. See below for examples of the failures and
how the fix handles these situations now.

I reported this issue earlier here:
     Intermittent problem with BogoMIPs calculation on Intel AP CPUs -
http://marc.info/?l=linux-kernel&m=129947246316875&w=4

I suspect this issue has been seen by others but as it is intermittent
and bogoMIPS for secondary CPUs are no longer printed out it might
have been difficult to identify this as the cause.  Perhaps these
unresolved issues, although quite old, might be relevant as possibly
this fault has been around for a while. In particular Case 1 may only
be relevant to 32bit kernels on newer HW (most people run 64bit
kernels?). Case 2 is less dramatic since the earlier fix in this area
and also intermittent.

   Re: bogomips discrepancy on Intel Core2 Quad CPU -
http://marc.info/?l=linux-kernel&m=118929277524298&w=4
   slow system and bogus bogomips  -
http://marc.info/?l=linux-kernel&m=116791286716107&w=4
   Re: Re: [RFC-PATCH] clocksource: update lpj if clocksource has -
http://marc.info/?l=linux-kernel&m=128952775819467&w=4

This issue is masked a little by commit
feae3203d711db0a9965300ee6d592257fdaae4f which only prints out the
first bogoMIPS value making it
much harder to notice other values differing. Perhaps it should be
changed to only suppress them when they are similar values?

Here are some outputs showing faults occurring and the new code
handling them properly. See my earlier message for examples of the
original failure.

    Case 1:   A Time Stamp Counter wrap:
...
Calibrating delay loop (skipped), value calculated using timer
frequency.. 6332.70 BogoMIPS (lpj=31663540)
....
calibrate_delay_direct() timer_rate_max=31666493
timer_rate_min=31666151 pre_start=4170369255 pre_end=4202035539
calibrate_delay_direct() timer_rate_max=2425955274
timer_rate_min=2425954941 pre_start=4265368533 pre_end=2396356387
calibrate_delay_direct() ignoring timer_rate as we had a TSC wrap
around start=4265368581 >=post_end=2396356511
calibrate_delay_direct() timer_rate_max=31666274
timer_rate_min=31665942 pre_start=2440373374 pre_end=2472039515
calibrate_delay_direct() timer_rate_max=31666492
timer_rate_min=31666160 pre_start=2535372139 pre_end=2567038422
calibrate_delay_direct() timer_rate_max=31666455
timer_rate_min=31666207 pre_start=2630371084 pre_end=2662037415
Calibrating delay using timer specific routine.. 6333.28 BogoMIPS (lpj=31666428)
Total of 2 processors activated (12665.99 BogoMIPS).
....

    Case 2:  Some thing (presumably the SMM interrupt?) causing the
very low increase in TSC counter for the DELAY_CALIBRATION_TICKS
increase in jiffies
...
Calibrating delay loop (skipped), value calculated using timer
frequency.. 6333.25 BogoMIPS (lpj=31666270)
...
calibrate_delay_direct() timer_rate_max=31666483
timer_rate_min=31666074 pre_start=4199536526 pre_end=4231202809
calibrate_delay_direct() timer_rate_max=864348 timer_rate_min=864016
pre_start=2405343672 pre_end=2406207897
calibrate_delay_direct() timer_rate_max=31666483
timer_rate_min=31666179 pre_start=2469540464 pre_end=2501206823
calibrate_delay_direct() timer_rate_max=31666511
timer_rate_min=31666122 pre_start=2564539400 pre_end=2596205712
calibrate_delay_direct() timer_rate_max=31666084
timer_rate_min=31665685 pre_start=2659538782 pre_end=2691204657
calibrate_delay_direct() dropping min bogoMips estimate 1 = 864348
Calibrating delay using timer specific routine.. 6333.27 BogoMIPS (lpj=31666390)
Total of 2 processors activated (12666.53 BogoMIPS).
...

After 70 boots I saw 2 variations <1% slip through

Andrew Worsley

View attachment "bogoMIPS.patch" of type "text/x-diff" (3224 bytes)

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