Re: [PATCH 2/8] powerpc/signal: Add unsafe_copy_{vsx, fpr}_from_user()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 08/02/2021 à 18:14, Christopher M. Riedl a écrit :
> On Sun Feb 7, 2021 at 4:12 AM CST, Christophe Leroy wrote:
>>
>>
>> Le 06/02/2021 à 18:39, Christopher M. Riedl a écrit :
>>> On Sat Feb 6, 2021 at 10:32 AM CST, Christophe Leroy wrote:
>>>>
>>>>
>>>> Le 20/10/2020 à 04:01, Christopher M. Riedl a écrit :
>>>>> On Fri Oct 16, 2020 at 10:48 AM CDT, Christophe Leroy wrote:
>>>>>>
>>>>>>
>>>>>> Le 15/10/2020 à 17:01, Christopher M. Riedl a écrit :
>>>>>>> Reuse the "safe" implementation from signal.c except for calling
>>>>>>> unsafe_copy_from_user() to copy into a local buffer. Unlike the
>>>>>>> unsafe_copy_{vsx,fpr}_to_user() functions the "copy from" functions
>>>>>>> cannot use unsafe_get_user() directly to bypass the local buffer since
>>>>>>> doing so significantly reduces signal handling performance.
>>>>>>
>>>>>> Why can't the functions use unsafe_get_user(), why does it significantly
>>>>>> reduces signal handling
>>>>>> performance ? How much significant ? I would expect that not going
>>>>>> through an intermediate memory
>>>>>> area would be more efficient
>>>>>>
>>>>>
>>>>> Here is a comparison, 'unsafe-signal64-regs' avoids the intermediate buffer:
>>>>>
>>>>> 	|                      | hash   | radix  |
>>>>> 	| -------------------- | ------ | ------ |
>>>>> 	| linuxppc/next        | 289014 | 158408 |
>>>>> 	| unsafe-signal64      | 298506 | 253053 |
>>>>> 	| unsafe-signal64-regs | 254898 | 220831 |
>>>>>
>>>>> I have not figured out the 'why' yet. As you mentioned in your series,
>>>>> technically calling __copy_tofrom_user() is overkill for these
>>>>> operations. The only obvious difference between unsafe_put_user() and
>>>>> unsafe_get_user() is that we don't have asm-goto for the 'get' variant.
>>>>> Instead we wrap with unsafe_op_wrap() which inserts a conditional and
>>>>> then goto to the label.
>>>>>
>>>>> Implemenations:
>>>>>
>>>>> 	#define unsafe_copy_fpr_from_user(task, from, label)   do {            \
>>>>> 	       struct task_struct *__t = task;                                 \
>>>>> 	       u64 __user *buf = (u64 __user *)from;                           \
>>>>> 	       int i;                                                          \
>>>>> 									       \
>>>>> 	       for (i = 0; i < ELF_NFPREG - 1; i++)                            \
>>>>> 		       unsafe_get_user(__t->thread.TS_FPR(i), &buf[i], label); \
>>>>> 	       unsafe_get_user(__t->thread.fp_state.fpscr, &buf[i], label);    \
>>>>> 	} while (0)
>>>>>
>>>>> 	#define unsafe_copy_vsx_from_user(task, from, label)   do {            \
>>>>> 	       struct task_struct *__t = task;                                 \
>>>>> 	       u64 __user *buf = (u64 __user *)from;                           \
>>>>> 	       int i;                                                          \
>>>>> 									       \
>>>>> 	       for (i = 0; i < ELF_NVSRHALFREG ; i++)                          \
>>>>> 		       unsafe_get_user(__t->thread.fp_state.fpr[i][TS_VSRLOWOFFSET], \
>>>>> 				       &buf[i], label);                        \
>>>>> 	} while (0)
>>>>>
>>>>
>>>> Do you have CONFIG_PROVE_LOCKING or CONFIG_DEBUG_ATOMIC_SLEEP enabled in
>>>> your config ?
>>>
>>> I don't have these set in my config (ppc64le_defconfig). I think I
>>> figured this out - the reason for the lower signal throughput is the
>>> barrier_nospec() in __get_user_nocheck(). When looping we incur that
>>> cost on every iteration. Commenting it out results in signal performance
>>> of ~316K w/ hash on the unsafe-signal64-regs branch. Obviously the
>>> barrier is there for a reason but it is quite costly.
>>
>> Interesting.
>>
>> Can you try with the patch I just sent out
>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/c72f014730823b413528e90ab6c4d3bcb79f8497.1612692067.git.christophe.leroy@csgroup.eu/
> 
> Yeah that patch solves the problem. Using unsafe_get_user() in a loop is
> actually faster on radix than using the intermediary buffer step. A
> summary of results below (unsafe-signal64-v6 uses unsafe_get_user() and
> avoids the local buffer):
> 
>          |                                  | hash   | radix  |
>          | -------------------------------- | ------ | ------ |
>          | unsafe-signal64-v5               | 194533 | 230089 |
>          | unsafe-signal64-v6               | 176739 | 202840 |
>          | unsafe-signal64-v5+barrier patch | 203037 | 234936 |
>          | unsafe-signal64-v6+barrier patch | 205484 | 241030 |
> 
> I am still expecting some comments/feedback on my v5 before sending out
> v6. Should I include your patch in my series as well?
> 

My patch is now flagged "under review" in patchwork so I suppose Michael picked it already.

Christophe