Subject: [PATCH v4 00/10] Improve signal performance on PPC64 with KUAP

From: "Christopher M. Riedl" <[email-masked]>
Date: Wed, 27 Jan 2021 22:04:14 -0600

As reported by Anton, there is a large penalty to signal handling
performance on radix systems using KUAP. The signal handling code
performs many user access operations, each of which needs to switch the
KUAP permissions bit to open and then close user access. This involves a
costly 'mtspr' operation [0].

There is existing work done on x86 and by Christopher Leroy for PPC32 to
instead open up user access in "blocks" using user_*_access_{begin,end}.
We can do the same in PPC64 to bring performance back up on KUAP-enabled
radix and now also hash MMU systems [1].

Hash MMU KUAP support along with uaccess flush has landed in linuxppc/next
since the last revision. This series also provides a large benefit on hash
with KUAP. However, in the hash implementation of KUAP the user AMR is
always restored during system_call_exception() which cannot be avoided.
Fewer user access switches naturally also result in less uaccess flushing.

The first two patches add some needed 'unsafe' versions of copy-from
functions. While these do not make use of asm-goto they still allow for
avoiding the repeated uaccess switches.

The third patch moves functions called by setup_sigcontext() into a new
prepare_setup_sigcontext() to simplify converting setup_sigcontext()
into an 'unsafe' version which assumes an open uaccess window later.

The fourth and fifths patches clean-up some of the Transactional Memory
ifdef stuff to simplify using uaccess blocks later.

The next two patches rewrite some of the signal64 helper functions to
be 'unsafe'. Finally, the last three patches update the main signal
handling functions to make use of the new 'unsafe' helpers and eliminate
some additional uaccess switching.

I used the will-it-scale signal1 benchmark to measure and compare
performance [2]. The below results are from running a minimal
kernel+initramfs QEMU/KVM guest on a POWER9 Blackbird:

	signal1_threads -t1 -s10

	|                             | hash   | radix  |
	| --------------------------- | ------ | ------ |
	| linuxppc/next               | 118693 | 133296 |
	| linuxppc/next w/o KUAP+KUEP | 228911 | 228654 |
	| unsafe-signal64             | 199443 | 234716 |

[0]: https://github.com/linuxppc/issues/issues/277
[1]: https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=196278
[2]: https://github.com/antonblanchard/will-it-scale/blob/master/tests/signal1.c

v4:	* Fix issues identified by Christophe Leroy (thanks for review)
	* Use __get_user() directly to copy the 8B sigset_t

v3:	* Rebase on latest linuxppc/next
	* Reword confusing commit messages
	* Add missing comma in macro in signal.h which broke compiles without
	  CONFIG_ALTIVEC
	* Validate hash KUAP signal performance improvements

v2:	* Rebase on latest linuxppc/next + Christophe Leroy's PPC32
	  signal series
	* Simplify/remove TM ifdefery similar to PPC32 series and clean
	  up the uaccess begin/end calls
	* Isolate non-inline functions so they are not called when
	  uaccess window is open

Christopher M. Riedl (8):
  powerpc/uaccess: Add unsafe_copy_from_user
  powerpc/signal: Add unsafe_copy_{vsx,fpr}_from_user()
  powerpc/signal64: Move non-inline functions out of setup_sigcontext()
  powerpc: Reference param in MSR_TM_ACTIVE() macro
  powerpc/signal64: Remove TM ifdefery in middle of if/else block
  powerpc/signal64: Replace setup_sigcontext() w/
    unsafe_setup_sigcontext()
  powerpc/signal64: Replace restore_sigcontext() w/
    unsafe_restore_sigcontext()
  powerpc/signal64: Use __get_user() to copy sigset_t

Daniel Axtens (2):
  powerpc/signal64: Rewrite handle_rt_signal64() to minimise uaccess
    switches
  powerpc/signal64: Rewrite rt_sigreturn() to minimise uaccess switches

 arch/powerpc/include/asm/reg.h     |   2 +-
 arch/powerpc/include/asm/uaccess.h |   3 +
 arch/powerpc/kernel/signal.h       |  33 ++++
 arch/powerpc/kernel/signal_64.c    | 251 ++++++++++++++++++-----------
 4 files changed, 196 insertions(+), 93 deletions(-)

-- 
2.26.1