linux-kernel - Re: Running an Ivy Bridge cpu at fixed frequency

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Date:   Thu, 5 Dec 2019 09:51:39 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     David Laight <David.Laight@...lab.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Running an Ivy Bridge cpu at fixed frequency



> On Dec 5, 2019, at 7:54 AM, David Laight <David.Laight@...lab.com> wrote:
> 
> From: Peter Zijlstra
>> Sent: 05 December 2019 09:46
>> As Andy already wrote, perf is really good for this.
>> Find attached, it probably is less shiny than what Andy handed you, but
>> contains all the bits required to frob something.
> 
> You are in a maze of incomplete documentation all disjoint.

I don’t see any documentation.  Maybe you shouldn’t have turned your flashlight on.

> 
> The x86 instruction set doc (eg 325462.pdf) defines the rdpmc instruction, tells you
> how many counters each cpu type has, but doesn't even contain a reference
> to how they are incremented.
> I guess there are some processor-specific MSR for that.
> 
> perf_event_open(2) tells you a few things, but doesn't actually what anything is.
> It contains all but the last 'if' clause of this function, without really saying
> what any of it does - or why you might do it this way.
> 
> static inline u64 mmap_read_self(void *addr)
> {
>       struct perf_event_mmap_page *pc = addr;
>       u32 seq, idx, time_mult = 0, time_shift = 0, width = 0;
>       u64 count, cyc = 0, time_offset = 0, enabled, running, delta;
>       s64 pmc = 0;
> 
>       do {
>               seq = pc->lock;
>               barrier();
> 
>               enabled = pc->time_enabled;
>               running = pc->time_running;
> 
>               if (pc->cap_user_time && enabled != running) {
>                       cyc = rdtsc();
>                       time_mult = pc->time_mult;
>                       time_shift = pc->time_shift;
>                       time_offset = pc->time_offset;
>               }
> 
>               idx = pc->index;
>               count = pc->offset;
>               if (pc->cap_user_rdpmc && idx) {
>                       width = pc->pmc_width;
>                       pmc = rdpmc(idx - 1);
>               }
> 
>               barrier();
>       } while (pc->lock != seq);
> 
>       if (idx) {
>               pmc <<= 64 - width;
>               pmc >>= 64 - width; /* shift right signed */
>               count += pmc;
>       }
> 
>       if (enabled != running) {
>               u64 quot, rem;
> 
>               quot = (cyc >> time_shift);
>               rem = cyc & ((1 << time_shift) - 1);
>               delta = time_offset + quot * time_mult +
>                       ((rem * time_mult) >> time_shift);
> 
>               enabled += delta;
>               if (idx)
>                       running += delta;
> 
>               quot = count / running;
>               rem = count % running;
>               count = quot * enabled + (rem * enabled) / running;
>       }
> 
>       return count;
> }
> 
> AFAICT:
> 1) The last clause is scaling the count up to allow for time when the hardware counter
>  couldn't be allocated.
>  I'm not convinced that is useful, better to ignore the entire measurement.
>  Half this got deleted from the man page, leaving strange 'set but unused' variables.
> 
> 2) The hardware counters are disabled while the process is asleep.
>  On wake a different pmc counter might be used (maybe on a different cpu).
>  The new cpu might not even have a counter available.
> 
> 3) If you don't want to scale up for missing periods it is probably enough to do:
>   do {
>       seq = pc->offset;
>       barrier();
>       idx = pc->index;
>       if (!index)
>           return -1;
>       count = pc->offset + rdpmc(idx - 1);
>   } while (seq != pc->seq);
>   return (unsigned int)count;
> 
> Not tried it yet :-)

Use my version :).  I just throw out the sample if we were preempted or if it was otherwise suspicious.

—Andy

> 
>   David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)