Date: Fri, 15 Mar 2019 06:57:16 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Mark Millard <marklmi@yahoo.com>, Bruce Evans <brde@optusnet.com.au>, freebsd-hackers Hackers <freebsd-hackers@freebsd.org>, FreeBSD PowerPC ML <freebsd-ppc@freebsd.org> Subject: Re: powerpc64 head -r344018 stuck sleeping problems: th->th_scale * tc_delta(th) overflows unsigned 64 bits sometimes [patched failed] Message-ID: <20190315064830.F7981@besplex.bde.org> In-Reply-To: <20190314193946.GJ2492@kib.kiev.ua> References: <20190303111931.GI68879@kib.kiev.ua> <20190303223100.B3572@besplex.bde.org> <20190303161635.GJ68879@kib.kiev.ua> <20190304043416.V5640@besplex.bde.org> <20190304114150.GM68879@kib.kiev.ua> <20190305031010.I4610@besplex.bde.org> <20190306172003.GD2492@kib.kiev.ua> <20190308001005.M2756@besplex.bde.org> <20190307222220.GK2492@kib.kiev.ua> <5EED3352-2E8C-4BEE-B281-4AC8DE9570C2@yahoo.com> <20190314193946.GJ2492@kib.kiev.ua>
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On Thu, 14 Mar 2019, Konstantin Belousov wrote:
> On Thu, Mar 07, 2019 at 05:29:51PM -0800, Mark Millard wrote:
>> A basic question and a small note.
>>
>> Question's context for it tc->tc_get_timecount(tc) values:
>>
>> In the powerpc64 context tc->tc_get_timecount(tc) is the lower
>> 32 bits of the tbr, in my context having a 33,333,333 MHz or so
>> increment rate for a machine with a 2.5 GHz or so clock rate.
>> The truncated 32 bit tbr value wraps every 128 seconds or so.
>> 2 sockets, 2 cores per socket, so 4 separate tbr values.
>>
>> The question is . . .
>>
>> In tc_delta's:
>>
>> tc->tc_get_timecount(tc) - th->th_offset_count
>>
>> is observing tc->tc_get_timecount(tc) < th->th_offset_count
>> ever supposed to be possible in correct operation, other than
>> tc->tc_get_timecount(tc) having wrapped around (and so being
>> newly 0 or "near" 0, no evidence of of having it having been
>> near 128 seconds or more for my context)?
> I think yes, there is no reason for current get_timecount() value
> to have any arithmetic relation to th_offset_count. Look at tc_windup()
> on how the th_offset_count is calculated. The final value is clamped
> by the tc_counter_mask, so only lower bits are important (higher bits
> are evacuated to th_offset or lost due to overflow if tc_windup()
> was not called soon enough).
Yes, it is a standard method to calculate time differences from a possibly-
wrapped counter as (finish - start) & mask in unsigned arithmetic, where
the counter must be checked before it wraps relative to 'start'.
>> The note:
>>
>> On 2019-Mar-7, at 14:22, Konstantin Belousov <kostikbel@gmail.com> wrote:
>>
>>> . . .
>>> +
>>> + if (__predict_false(delta < large_delta)) {
>>
>> I thought that delta<large_delta was the non-overflow context
>> for scale*delta and that the overflow case for the multiplication
>> was when delta>=large_delta .
> You are right, I fixed this in my repo.
>>
>>> + /* Avoid overflow for scale * delta. */
>>> + x = (scale >> 32) * delta;
>>> + bt->sec += x >> 32;
>>> + bintime_addx(bt, x << 32);
>>> + bintime_addx(bt, (scale & 0xffffffff) * delta);
>>> + } else {
>>> + bintime_addx(bt, scale * delta);
>>> + }
>>> . . .
Fixed in my version too.
I might have helped break this. I reversed the condition to get the
unusual path executed (though not when it overflow), and forgot to
undo this. At least the unusual path got checked more).
Bruce
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