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.
This also explains why the copy_{fpr,vsx}_to_user() direction does not
suffer from the slowdown because there is no need for barrier_nospec().
>
> If yes, could you try together with the patch from Alexey
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20210204121612.32721-1-...@ozlabs.ru/
> ?
>
> Thanks
> Christophe
