linux-kernel - Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS

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Message-ID: <ZPg8pJR55pmEZ4vQ@ysun46-mobl.ccr.corp.intel.com>
Date:   Wed, 6 Sep 2023 16:47:32 +0800
From:   Yi Sun <yi.sun@...el.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     <dave.hansen@...el.com>, <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <sohil.mehta@...el.com>, <ak@...ux.intel.com>,
        <ilpo.jarvinen@...ux.intel.com>, <heng.su@...el.com>,
        <tony.luck@...el.com>, <dave.hansen@...ux.intel.com>,
        <yi.sun@...el.intel.com>
Subject: Re: [PATCH v6 1/3] x86/fpu: Measure the Latency of XSAVES and XRSTORS

On 02.09.2023 12:49, Ingo Molnar wrote:
>
>* Yi Sun <yi.sun@...el.com> wrote:
>
>> +#define XSTATE_XSAVE(fps, lmask, hmask, err)				\
>> +	do {								\
>> +		struct fpstate *f = fps;				\
>> +		u64 tc = -1;						\
>> +		if (xsave_tracing_enabled())				\
>> +			tc = trace_clock();				\
>> +		__XSTATE_XSAVE(&f->regs.xsave, lmask, hmask, err);	\
>> +		if (xsave_tracing_enabled())				\
>> +			trace_x86_fpu_latency_xsave(f, trace_clock() - tc);\
>> +	} while (0)
>> +
>>  /*
>>   * Use XRSTORS to restore context if it is enabled. XRSTORS supports compact
>>   * XSAVE area format.
>>   */
>> -#define XSTATE_XRESTORE(st, lmask, hmask)				\
>> +#define __XSTATE_XRESTORE(st, lmask, hmask)				\
>>  	asm volatile(ALTERNATIVE(XRSTOR,				\
>>  				 XRSTORS, X86_FEATURE_XSAVES)		\
>>  		     "\n"						\
>> @@ -140,6 +168,17 @@ static inline u64 xfeatures_mask_independent(void)
>>  		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
>>  		     : "memory")
>>
>> +#define XSTATE_XRESTORE(fps, lmask, hmask)				\
>> +	do {								\
>> +		struct fpstate *f = fps;				\
>> +		u64 tc = -1;						\
>> +		if (xrstor_tracing_enabled())				\
>> +			tc = trace_clock();				\
>> +		__XSTATE_XRESTORE(&f->regs.xsave, lmask, hmask);	\
>> +		if (xrstor_tracing_enabled())				\
>> +			trace_x86_fpu_latency_xrstor(f, trace_clock() - tc);\
>> +	} while (0)
>> +
>>  #if defined(CONFIG_X86_64) && defined(CONFIG_X86_DEBUG_FPU)
>>  extern void xfd_validate_state(struct fpstate *fpstate, u64 mask, bool rstor);
>>  #else
>> @@ -184,7 +223,7 @@ static inline void os_xsave(struct fpstate *fpstate)
>>  	WARN_ON_FPU(!alternatives_patched);
>>  	xfd_validate_state(fpstate, mask, false);
>>
>> -	XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);
>> +	XSTATE_XSAVE(fpstate, lmask, hmask, err);
>>
>>  	/* We should never fault when copying to a kernel buffer: */
>>  	WARN_ON_FPU(err);
>> @@ -201,7 +240,7 @@ static inline void os_xrstor(struct fpstate *fpstate, u64 mask)
>>  	u32 hmask = mask >> 32;
>>
>>  	xfd_validate_state(fpstate, mask, true);
>> -	XSTATE_XRESTORE(&fpstate->regs.xsave, lmask, hmask);
>> +	XSTATE_XRESTORE(fpstate, lmask, hmask);
>>  }
>
>Instead of adding overhead to the regular FPU context saving/restoring code
>paths, could you add a helper function that has tracing code included, but
>which isn't otherwise used - and leave the regular code with no tracing
>overhead?
>
>This puts a bit of a long-term maintenance focus on making sure that the
>traced functionality won't bitrot, but I'd say that's preferable to adding
>tracing overhead.
>
Hi Ingo,
It is actually a helper function, and I have renamed the original function
to __XSTATE_XSAVE. This function is only used in this particular context.

Furthermore, according doc static-keys.txt, the condition
xrstor_tracing_enabled() would introduce only a minimal overhead when the
trace is disabled. I believe it is a negligible impact on the performance
when the trace is disabled.

Please let me know if you have any questions or concerns about this function.

Thanks
   --Sun, Yi