Re: [PATCH v5 3/3] power: amd power monitor support

[Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>]

25/08/2023 17:00, Tyler Retzlaff пишет:
> On Thu, Aug 24, 2023 at 10:04:42AM +0100, Ferruh Yigit wrote:
>> On 8/23/2023 5:03 PM, Tyler Retzlaff wrote:
>>> On Wed, Aug 23, 2023 at 10:19:39AM +0100, Ferruh Yigit wrote:
>>>> On 8/22/2023 11:30 PM, Konstantin Ananyev wrote:
>>>>> 18/08/2023 14:48, Bruce Richardson пишет:
>>>>>> On Fri, Aug 18, 2023 at 02:25:14PM +0100, Ferruh Yigit wrote:
>>>>>>> On 8/17/2023 3:18 PM, Konstantin Ananyev wrote:
>>>>>>>>
>>>>>>>>>> Caution: This message originated from an External Source. Use
>>>>>>>>>> proper caution
>>>>>>>>>> when opening attachments, clicking links, or responding.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Wed, Aug 16, 2023 at 11:59:59AM -0700, Sivaprasad Tummala wrote:
>>>>>>>>>>> mwaitx allows EPYC processors to enter a implementation dependent
>>>>>>>>>>> power/performance optimized state (C1 state) for a specific
>>>>>>>>>>> period or
>>>>>>>>>>> until a store to the monitored address range.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Sivaprasad Tummala <sivaprasad.tummala@amd.com>
>>>>>>>>>>> Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>
>>>>>>>>>>> ---
>>>>>>>>>>>    lib/eal/x86/rte_power_intrinsics.c | 77
>>>>>>>>>>> +++++++++++++++++++++++++-----
>>>>>>>>>>>    1 file changed, 66 insertions(+), 11 deletions(-)
>>>>>>>>>>>
>>>>>>>>>>> diff --git a/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>>>> b/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>>>> index 6eb9e50807..b4754e17da 100644
>>>>>>>>>>> --- a/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>>>> +++ b/lib/eal/x86/rte_power_intrinsics.c
>>>>>>>>>>> @@ -17,6 +17,60 @@ static struct power_wait_status {
>>>>>>>>>>>         volatile void *monitor_addr; /**< NULL if not currently
>>>>>>>>>>> sleeping
>>>>>>>>>>> */  } __rte_cache_aligned wait_status[RTE_MAX_LCORE];
>>>>>>>>>>>
>>>>>>>>>>> +/**
>>>>>>>>>>> + * These functions uses UMONITOR/UMWAIT instructions and will
>>>>>>>>>>> enter C0.2
>>>>>>>>>> state.
>>>>>>>>>>> + * For more information about usage of these instructions, please
>>>>>>>>>>> +refer to
>>>>>>>>>>> + * Intel(R) 64 and IA-32 Architectures Software Developer's Manual.
>>>>>>>>>>> + */
>>>>>>>>>>> +static void intel_umonitor(volatile void *addr) {
>>>>>>>>>>> +     /* UMONITOR */
>>>>>>>>>>> +     asm volatile(".byte 0xf3, 0x0f, 0xae, 0xf7;"
>>>>>>>>>>> +                     :
>>>>>>>>>>> +                     : "D"(addr));
>>>>>>>>>>> +}
>>>>>>>>>>> +
>>>>>>>>>>> +static void intel_umwait(const uint64_t timeout) {
>>>>>>>>>>> +     const uint32_t tsc_l = (uint32_t)timeout;
>>>>>>>>>>> +     const uint32_t tsc_h = (uint32_t)(timeout >> 32);
>>>>>>>>>>> +     /* UMWAIT */
>>>>>>>>>>> +     asm volatile(".byte 0xf2, 0x0f, 0xae, 0xf7;"
>>>>>>>>>>> +                     : /* ignore rflags */
>>>>>>>>>>> +                     : "D"(0), /* enter C0.2 */
>>>>>>>>>>> +                     "a"(tsc_l), "d"(tsc_h)); }
>>>>>>>>>>
>>>>>>>>>> question and perhaps Anatoly Burakov can chime in with expertise.
>>>>>>>>>>
>>>>>>>>>> gcc/clang have built-in intrinsics for umonitor and umwait i
>>>>>>>>>> believe as per our other
>>>>>>>>>> thread of discussion is there a benefit to also providing inline
>>>>>>>>>> assembly over just
>>>>>>>>>> using the intrinsics? I understand that the intrinsics may not
>>>>>>>>>> exist for the monitorx
>>>>>>>>>> and mwaitx below so it is probably necessary for amd.
>>>>>>>>>>
>>>>>>>>>> so the suggestion here is when they are available just use the
>>>>>>>>>> intrinsics.
>>>>>>>>>>
>>>>>>>>>> thanks
>>>>>>>>>>
>>>>>>>>> The gcc built-in functions
>>>>>>>>> __builtin_ia32_monitorx()/__builtin_ia32_mwaitx are available only
>>>>>>>>> when -mmwaitx
>>>>>>>>> is used specific for AMD platforms. On generic builds, these
>>>>>>>>> built-ins are not available and hence inline
>>>>>>>>> assembly is required here.
>>>>>>>>
>>>>>>>> Ok... but we can probably put them into a separate .c file that will
>>>>>>>> be compiled with that specific flag?
>>>>>>>> Same thing can be probably done for Intel specific instructions.
>>>>>>>> In general, I think it is much more preferable to use built-ins vs
>>>>>>>> inline assembly
>>>>>>>> (if possible off-course).
>>>>>>>>
>>>>>>>
>>>>>>> We don't compile different set of files for AMD and Intel, but there are
>>>>>>> runtime checks, so putting into separate file is not much different.
>>>>>
>>>>> Well, we probably don't compile .c files for particular vendor, but we
>>>>> definitely do compile some .c files for particular ISA extensions.
>>>>> Let say there are files in lib/acl that requires various '-mavx512*'
>>>>> flags, same for other libs and PMDs.
>>>>> So still not clear to me why same approach can't be applied to
>>>>> power_instrincts.c?
>>>>>
>>>>>>>
>>>>>>> It may be an option to always enable compiler flag (-mmwaitx), I think
>>>>>>> it won't hurt other platforms but I am not sure about implications of
>>>>>>> this to other platforms (what was the motivation for the compiler guys
>>>>>>> to enable these build-ins with specific flag?).
>>>>>>>
>>>>>>> Also this requires detecting compiler that supports 'mmwaitx' or not,
>>>>>>> etc..
>>>>>>>
>>>>>> This is the biggest reason why we have in the past added support for
>>>>>> these
>>>>>> instructions via asm bytes rather than intrinsics. It takes a long
>>>>>> time for
>>>>>> end-user compilers, especially those in LTS releases, to get the
>>>>>> necessary
>>>>>> intrinsics.
>>>>>
>>>>> Yep, understand.
>>>>> But why then we can't have both implementations?
>>>>> Let say if WAITPKG is defined we can use builtins for
>>>>> umonitor/umwait/tpause, otherwise we fallback to inline asm implementation.
>>>>> Same story for MWAITX/monitorx.
>>>>>
>>>>
>>>> Yes this can be done,
>>>> it can be done either as different .c files per implementation, or as
>>>> #ifdef in same file.
>>>>
>>>> But eventually asm implementation is required, as fallback, and if we
>>>> will rely on asm implementation anyway, does it worth to have the
>>>> additional checks to be able to use built-in intrinsic?
>>>>
>>>> Does it helps to comment name of the built-in function to inline
>>>> assembly code, to document intention and another possible implementation?
>>>
>>> the main value of preferring intrinsics is that when they are available
>>> they also work with msvc/windows. the msvc toolchain does not support
>>> inline asm. so some of the targets have to use intrinsics because that's all
>>> there is.
>>>
>>
>> How windows handles current power APIs without inline asm support, like
>> rte_power_intrinsics.c one?
> 
> so this is a windows vs toolchain entanglement.
> 
>> Also will using both built-in and inline assembly work for Windows,
>> since there may be compiler versions that doesn't support built-in
>> functions, they should disable APIs altogether, and this can create a
>> scenario that list of exposed APIs changes based on compiler version.
> 
> so I don't intend to disable apis, theres usually a way to make them
> work and there should not be any api changes when done correctly.
> 
> windows/clang/mingw
>      * inline asm may be used, but for me isn't preferred
> 
> windows/msvc
>      * intrinsics (when available)
>      * non-inline asm in a .s (when no intrinsics available)
>      * keeping in mind that the compiler version isn't tied to windows
>        OS release so it's easier for me to document that you need a
>        newer compiler arbitrarily. The periods where there are no intrinsics
>        end up being short-lived.
>     
> I'm on the hook for windows/msvc any stickyness dealing with it ends up
> being my problem.

As I can read rte_power_instrintcts.c, for each set of power instructions
we have related static variable: wait*_supported.
So if we need to support compiler that supports neither
new bultins nor inline-asm, then it probably possible to rearrange the 
code to keep these static vars equal zero for such case.


> 
>>
>>>>
>>>>>> Consider a user running e.g. RHEL 8, who wants to take
>>>>>> advantages of the latest DPDK features; they should not be required to
>>>>>> upgrade their compiler - and possibly binutils/assembler - to do so.
>>>>>>
>>>>>> /Bruce
>>>>>