svn commit: r336025 - in head/sys: amd64/include i386/include
Jeff Roberson
jroberson at jroberson.net
Fri Jul 6 22:58:54 UTC 2018
On Fri, 6 Jul 2018, Rodney W. Grimes wrote:
>> On Fri, Jul 6, 2018, 12:27 PM Rodney W. Grimes <
>> freebsd at pdx.rh.cn85.dnsmgr.net> wrote:
>>
>>>> On Fri, Jul 6, 2018 at 9:52 AM, Rodney W. Grimes <
>>>> freebsd at pdx.rh.cn85.dnsmgr.net> wrote:
>>>>
>>>>>> On Fri, Jul 6, 2018 at 9:32 AM, Rodney W. Grimes <
>>>>>> freebsd at pdx.rh.cn85.dnsmgr.net> wrote:
>>>>>>
>>>>>>>> Author: hselasky
>>>>>>>> Date: Fri Jul 6 10:13:42 2018
>>>>>>>> New Revision: 336025
>>>>>>>> URL: https://svnweb.freebsd.org/changeset/base/336025
>>>>>>>>
>>>>>>>> Log:
>>>>>>>> Make sure kernel modules built by default are portable between
>>> UP
>>>>> and
>>>>>>>> SMP systems by extending defined(SMP) to include
>>>>> defined(KLD_MODULE).
>>>>>>>>
>>>>>>>> This is a regression issue after r335873 .
>>>>>>>>
>>>>>>>> Discussed with: mmacy@
>>>>>>>> Sponsored by: Mellanox Technologies
>>>>>>>
>>>>>>> Though this fixes the issue, it also means that now when
>>>>>>> anyone intentionally builds a UP kernel with modules
>>>>>>> they are getting SMP support in the modules and I am
>>>>>>> not sure they would want that. I know I don't.
>>>>>>>
>>>>>>
>>>>>>
>>>>>> On UP systems, these additional opcodes are harmless. They take a few
>>>>> extra
>>>>>> cycles (since they lock an uncontested bus) and add a couple extra
>>> memory
>>>>>> barriers (which will be NOPs). On MP systems, atomics now work by
>>>>> default.
>>>>>> Had we not defaulted like this, all modules built outside of a kernel
>>>>> build
>>>>>> env would have broken atomics. Given that (a) the overwhelming
>>> majority
>>>>>> (99% or more) is SMP and (b) the MP code merely adds a few cycles to
>>>>> what's
>>>>>> already a not-too-expensive operation, this was the right choice.
>>>>>>
>>>>>> It simply doesn't matter for systems that are relevant to the project
>>>>>> today. While one could try to optimize this a little (for example, by
>>>>>> having SMP defined to be 0 or 1, say, and changing all the ifdef SMP
>>> to
>>>>> if
>>>>>> (defined(SMP) && SMP != 0)), it's likely not going to matter enough
>>> for
>>>>>> anybody to make the effort. UP on x86 is simply not relevant enough
>>> to
>>>>>> optimize for it. Even in VMs, people run SMP kernels typically even
>>> when
>>>>>> they just allocate one CPU to the VM.
>>>>>>
>>>>>> So while we still support the UP config, and we'll let people build
>>>>>> optimized kernels for x86, we've flipped the switch from pessimized
>>> for
>>>>> SMP
>>>>>> modules to pessimized for UP modules, which seems like quite the
>>>>> reasonable
>>>>>> trade-off.
>>>>>>
>>>>>> Were it practical to do so, I'd suggest de-orbiting UP on x86.
>>> However,
>>>>>> it's a lot of work for not much benefit and we'd need to invent much
>>>>> crazy
>>>>>> to get there.
>>>>>
>>>>> Trivial to fix this with
>>>>> +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE) ||
>>>>> !defined(KLD_UP_MODULES)
>>>>
>>>>
>>>> Nope. Not so trivial. Who defines KLD_UP_MODULES?
>>>
>>> Call it SMP_KLD_MODULES, and it gets defined the same place SMP does.
>>>
>>
>> Not so simple. SMP is defined in the config file, and winds up in one of
> No problem, that is where I would be defining this anyway, or in the
> latest case removing it and SMP for my UP kernel build.
>
>> the option files. It will be absent for stand alone builds,
> I am ok with that. And it would be reasonable to default to SMP.
>
>> though. These
>> change tweak the default yo be inlined and to include the sequence that
>> works everywhere.
>>
>>>
>>>> And really, it's absolutely not worth it unless someone shows up with
>>>> numbers to show the old 'function call to optimal routine' is actually
>>>> faster than the new 'inline to slightly unoptimal code'. Since I think
>>> the
>>>> function call overhead is larger than the pessmizations, I'm not sure
>>> what
>>>> the fuss is about.
>>>
>>> I have no issues with the SMP converting from function calls to
>>> inline locks, I just want to retain the exact same code I had
>>> before any of these changes, and that was A UP built system
>>> without any SMP locking. Is it too much to ask to keep what
>>> already worked?
>>>
>>
>> This doesn't enable or disable locks in the muted sense. It just changes
>> the atomic ops for the kernel from a function call to an inlined function.
>> The inlining is more efficient than the call, even with the overhead added
>> by always inlining the same stuff. It still is faster than before.
>>
>> And userland has done this forever...
>>
>> So I honestly think even UP builds are better off, even if it's not hyper
>> optimized for UP. The lock instruction prefix is minimal overhead (a cycle
>> I think).
>
> I do not believe, and Bruce seems to have evidence, that LOCK is not
> a one cycle cost. And in my head I know that it can not be that
> simple as it causes lots of very special things to happen in the
> pipeline to ensure you are locked.
It is definitely not one cycle. It is a couple dozen depending on the
machine. You can benchmark yourself with the attached patch.
Here's two atomics in a loop with and without lock on a Zen:
desktop# ./nonatomic
0 8838: 8 ns
desktop# ./atomic
0 32887: 32 ns
This is at ~3ghz so three instructions per-ns. Cut this in half for
per-atomic cost. 12ns of overhead or 36 cycles. One of them is a fetchadd
so there is extra cost associated with that. This is also worst case
because the machine can't execute anything other than the atomics. If
there were other instructions that didn't operate on overlapping memory
addresses they could run in parallel.
Jeff
>
>> This is different than the mutexes we optimize for the UP cases
>> (and which aren't affected by this change). It's really not a big deal.
>
> CPU's are not getting any faster, cycles are cycles, and I think we
> should at least investigate further before we just start making
> assumptions about the lock prefix being a 1 cycle cheap thing to
> do.
>
>>
>>>>>
>>>>>>>> Modified:
>>>>>>>> head/sys/amd64/include/atomic.h
>>>>>>>> head/sys/i386/include/atomic.h
>>>>>>>>
>>>>>>>> Modified: head/sys/amd64/include/atomic.h
>>>>>>>> ============================================================
>>>>>>> ==================
>>>>>>>> --- head/sys/amd64/include/atomic.h Fri Jul 6 10:10:00 2018
>>>>>>> (r336024)
>>>>>>>> +++ head/sys/amd64/include/atomic.h Fri Jul 6 10:13:42 2018
>>>>>>> (r336025)
>>>>>>>> @@ -132,7 +132,7 @@ void atomic_store_rel_##TYPE(
>>>>> volatile
>>>>>>> u_##TYPE *p, u_
>>>>>>>> * For userland, always use lock prefixes so that the binaries
>>> will
>>>>> run
>>>>>>>> * on both SMP and !SMP systems.
>>>>>>>> */
>>>>>>>> -#if defined(SMP) || !defined(_KERNEL)
>>>>>>>> +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE)
>>>>>>>> #define MPLOCKED "lock ; "
>>>>>>>> #else
>>>>>>>> #define MPLOCKED
>>>>>>>> @@ -354,7 +354,7 @@ atomic_testandclear_long(volatile u_long *p,
>>>>> u_int
>>>>>>> v)
>>>>>>>> */
>>>>>>>> #define OFFSETOF_MONITORBUF 0x100
>>>>>>>>
>>>>>>>> -#if defined(SMP)
>>>>>>>> +#if defined(SMP) || defined(KLD_MODULE)
>>>>>>>> static __inline void
>>>>>>>> __storeload_barrier(void)
>>>>>>>> {
>>>>>>>>
>>>>>>>> Modified: head/sys/i386/include/atomic.h
>>>>>>>> ============================================================
>>>>>>> ==================
>>>>>>>> --- head/sys/i386/include/atomic.h Fri Jul 6 10:10:00 2018
>>>>>>> (r336024)
>>>>>>>> +++ head/sys/i386/include/atomic.h Fri Jul 6 10:13:42 2018
>>>>>>> (r336025)
>>>>>>>> @@ -143,7 +143,7 @@ void atomic_subtract_64(volatile
>>>>>>> uint64_t *, uint64_t
>>>>>>>> * For userland, always use lock prefixes so that the binaries
>>> will
>>>>> run
>>>>>>>> * on both SMP and !SMP systems.
>>>>>>>> */
>>>>>>>> -#if defined(SMP) || !defined(_KERNEL)
>>>>>>>> +#if defined(SMP) || !defined(_KERNEL) || defined(KLD_MODULE)
>>>>>>>> #define MPLOCKED "lock ; "
>>>>>>>> #else
>>>>>>>> #define MPLOCKED
>>>>>>>> @@ -302,7 +302,7 @@ atomic_testandclear_int(volatile u_int *p,
>>>>> u_int v)
>>>>>>>> */
>>>>>>>>
>>>>>>>> #if defined(_KERNEL)
>>>>>>>> -#if defined(SMP)
>>>>>>>> +#if defined(SMP) || defined(KLD_MODULE)
>>>>>>>> #define __storeload_barrier() __mbk()
>>>>>>>> #else /* _KERNEL && UP */
>>>>>>>> #define __storeload_barrier() __compiler_membar()
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Rod Grimes
>>>>>>> rgrimes at freebsd.org
>>>>>>>
>>>>>>>
>>>>>
>>>>> --
>>>>> Rod Grimes
>>>>> rgrimes at freebsd.org
>>>>>
>>>
>>> --
>>> Rod Grimes
>>> rgrimes at freebsd.org
>>>
>
> --
> Rod Grimes rgrimes at freebsd.org
>
-------------- next part --------------
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#define ITER 1000000
static __inline u_int
atomic_fetchadd_int(volatile u_int *p, u_int v)
{
__asm __volatile(
" lock xaddl %0,%1 ; "
"# atomic_fetchadd_int"
: "+r" (v), /* 0 */
"+m" (*p) /* 1 */
: : "cc");
return (v);
}
static __inline void
atomic_add_int(volatile u_int *p, u_int v)
{
__asm __volatile(
" lock addl %0,%1 ; "
: "+r" (v), /* 0 */
"+m" (*p) /* 1 */
: : "cc");
}
int
main(int argc, char **argv)
{
struct timeval start, end;
volatile int foo;
uint64_t ns;
int i;
gettimeofday(&start, NULL);
for (i = 0; i < ITER; i++) {
atomic_add_int(&foo, 1);
atomic_fetchadd_int(&foo, -1);
}
gettimeofday(&end, NULL);
end.tv_sec -= start.tv_sec;
end.tv_usec -= start.tv_usec;
ns = end.tv_usec / (ITER / 1000);
printf("%d %d: %d ns\n", end.tv_sec, end.tv_usec, ns);
}
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