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.le...@csgroup.eu/
Thanks
Christophe
