kernel/arch/powerpc/include/asm/membarrier.h, branch linux-6.2.y

powerpc/non-smp: Unconditionaly call smp_mb() on switch_mm

2021-08-10T13:14:55Z

Commit 3ccfebedd8cf ("powerpc, membarrier: Skip memory barrier in switch_mm()") added some logic to skip the smp_mb() in switch_mm_irqs_off() before the call to switch_mmu_context(). However, on non SMP smp_mb() is just a compiler barrier and doing it unconditionaly is simpler than the logic used to check whether the barrier is needed or not. After the patch: 00000000 : ... c: 7c 04 18 40 cmplw r4,r3 10: 81 24 00 24 lwz r9,36(r4) 14: 91 25 04 c8 stw r9,1224(r5) 18: 4d 82 00 20 beqlr 1c: 48 00 00 00 b 1c 1c: R_PPC_REL24 switch_mmu_context Before the patch: 00000000 : ... c: 7c 04 18 40 cmplw r4,r3 10: 81 24 00 24 lwz r9,36(r4) 14: 91 25 04 c8 stw r9,1224(r5) 18: 4d 82 00 20 beqlr 1c: 81 24 00 28 lwz r9,40(r4) 20: 71 29 00 0a andi. r9,r9,10 24: 40 82 00 34 bne 58 28: 48 00 00 00 b 28 28: R_PPC_REL24 switch_mmu_context ... 58: 2c 03 00 00 cmpwi r3,0 5c: 41 82 ff cc beq 28 60: 48 00 00 00 b 60 60: R_PPC_REL24 switch_mmu_context Signed-off-by: Christophe Leroy Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/e9d501da0c59f60ca767b1b3ea4603fce6d02b9e.1625486440.git.christophe.leroy@csgroup.eu

membarrier: Provide GLOBAL_EXPEDITED command

2018-02-05T20:34:31Z

Allow expedited membarrier to be used for data shared between processes through shared memory. Processes wishing to receive the membarriers register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. Those which want to issue membarrier invoke MEMBARRIER_CMD_GLOBAL_EXPEDITED. This allows extremely simple kernel-level implementation: we have almost everything we need with the PRIVATE_EXPEDITED barrier code. All we need to do is to add a flag in the mm_struct that will be used to check whether we need to send the IPI to the current thread of each CPU. There is a slight downside to this approach compared to targeting specific shared memory users: when performing a membarrier operation, all registered "global" receivers will get the barrier, even if they don't share a memory mapping with the sender issuing MEMBARRIER_CMD_GLOBAL_EXPEDITED. This registration approach seems to fit the requirement of not disturbing processes that really deeply care about real-time: they simply should not register with MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED. In order to align the membarrier command names, the "MEMBARRIER_CMD_SHARED" command is renamed to "MEMBARRIER_CMD_GLOBAL", keeping an alias of MEMBARRIER_CMD_SHARED to MEMBARRIER_CMD_GLOBAL for UAPI header backward compatibility. Signed-off-by: Mathieu Desnoyers Acked-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Cc: Andrea Parri Cc: Andrew Hunter Cc: Andy Lutomirski Cc: Avi Kivity Cc: Benjamin Herrenschmidt Cc: Boqun Feng Cc: Dave Watson Cc: David Sehr Cc: Greg Hackmann Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Maged Michael Cc: Michael Ellerman Cc: Paul E. McKenney Cc: Paul Mackerras Cc: Russell King Cc: Will Deacon Cc: linux-api@vger.kernel.org Link: http://lkml.kernel.org/r/20180129202020.8515-5-mathieu.desnoyers@efficios.com Signed-off-by: Ingo Molnar

powerpc, membarrier: Skip memory barrier in switch_mm()

2018-02-05T20:34:02Z

Allow PowerPC to skip the full memory barrier in switch_mm(), and only issue the barrier when scheduling into a task belonging to a process that has registered to use expedited private. Threads targeting the same VM but which belong to different thread groups is a tricky case. It has a few consequences: It turns out that we cannot rely on get_nr_threads(p) to count the number of threads using a VM. We can use (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1) instead to skip the synchronize_sched() for cases where the VM only has a single user, and that user only has a single thread. It also turns out that we cannot use for_each_thread() to set thread flags in all threads using a VM, as it only iterates on the thread group. Therefore, test the membarrier state variable directly rather than relying on thread flags. This means membarrier_register_private_expedited() needs to set the MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows private expedited membarrier commands to succeed. membarrier_arch_switch_mm() now tests for the MEMBARRIER_STATE_PRIVATE_EXPEDITED flag. Signed-off-by: Mathieu Desnoyers Acked-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Cc: Alan Stern Cc: Alexander Viro Cc: Andrea Parri Cc: Andrew Hunter Cc: Andy Lutomirski Cc: Avi Kivity Cc: Benjamin Herrenschmidt Cc: Boqun Feng Cc: Dave Watson Cc: David Sehr Cc: Greg Hackmann Cc: Linus Torvalds Cc: Maged Michael Cc: Michael Ellerman Cc: Nicholas Piggin Cc: Paul E. McKenney Cc: Paul Mackerras Cc: Russell King Cc: Will Deacon Cc: linux-api@vger.kernel.org Cc: linux-arch@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org Link: http://lkml.kernel.org/r/20180129202020.8515-3-mathieu.desnoyers@efficios.com Signed-off-by: Ingo Molnar