Re: [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code

[Nicholas Piggin]

Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm:
> The core scheduler isn't a great place for
> membarrier_mm_sync_core_before_usermode() -- the core scheduler
> doesn't actually know whether we are lazy.  With the old code, if a
> CPU is running a membarrier-registered task, goes idle, gets unlazied
> via a TLB shootdown IPI, and switches back to the
> membarrier-registered task, it will do an unnecessary core sync.

I don't really mind, but ARM64 at least hints they might need it
at some point. They can always add it back then, but let's check.

> Conveniently, x86 is the only architecture that does anything in this
> sync_core_before_usermode(), so membarrier_mm_sync_core_before_usermode()
> is a no-op on all other architectures and we can just move the code.

If ARM64 does want it (now or later adds it back), x86 can always make 
the membarrier_mm_sync_core_before_usermode() a nop with comment 
explaining where it executes the serializing instruction.

I'm fine with the patch though, except I would leave the comment in the
core sched code saying any arch specific sequence to deal with
SYNC_CORE is required for that case.

Thanks,
Nick

> 
> (I am not claiming that the SYNC_CORE code was correct before or after this
>  change on any non-x86 architecture.  I merely claim that this change
>  improves readability, is correct on x86, and makes no change on any other
>  architecture.)
> 
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/mm/tlb.c        | 53 +++++++++++++++++++++++++++++++---------
>  include/linux/sched/mm.h | 13 ----------
>  kernel/sched/core.c      | 13 ++++------
>  3 files changed, 46 insertions(+), 33 deletions(-)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 78804680e923..59488d663e68 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -8,6 +8,7 @@
>  #include <linux/export.h>
>  #include <linux/cpu.h>
>  #include <linux/debugfs.h>
> +#include <linux/sched/mm.h>
>  
>  #include <asm/tlbflush.h>
>  #include <asm/mmu_context.h>
> @@ -473,16 +474,24 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  		this_cpu_write(cpu_tlbstate_shared.is_lazy, false);
>  
>  	/*
> -	 * The membarrier system call requires a full memory barrier and
> -	 * core serialization before returning to user-space, after
> -	 * storing to rq->curr, when changing mm.  This is because
> -	 * membarrier() sends IPIs to all CPUs that are in the target mm
> -	 * to make them issue memory barriers.  However, if another CPU
> -	 * switches to/from the target mm concurrently with
> -	 * membarrier(), it can cause that CPU not to receive an IPI
> -	 * when it really should issue a memory barrier.  Writing to CR3
> -	 * provides that full memory barrier and core serializing
> -	 * instruction.
> +	 * membarrier() support requires that, when we change rq->curr->mm:
> +	 *
> +	 *  - If next->mm has membarrier registered, a full memory barrier
> +	 *    after writing rq->curr (or rq->curr->mm if we switched the mm
> +	 *    without switching tasks) and before returning to user mode.
> +	 *
> +	 *  - If next->mm has SYNC_CORE registered, then we sync core before
> +	 *    returning to user mode.
> +	 *
> +	 * In the case where prev->mm == next->mm, membarrier() uses an IPI
> +	 * instead, and no particular barriers are needed while context
> +	 * switching.
> +	 *
> +	 * x86 gets all of this as a side-effect of writing to CR3 except
> +	 * in the case where we unlazy without flushing.
> +	 *
> +	 * All other architectures are civilized and do all of this implicitly
> +	 * when transitioning from kernel to user mode.
>  	 */
>  	if (real_prev == next) {
>  		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> @@ -500,7 +509,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  		/*
>  		 * If the CPU is not in lazy TLB mode, we are just switching
>  		 * from one thread in a process to another thread in the same
> -		 * process. No TLB flush required.
> +		 * process. No TLB flush or membarrier() synchronization
> +		 * is required.
>  		 */
>  		if (!was_lazy)
>  			return;
> @@ -510,16 +520,35 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  		 * If the TLB is up to date, just use it.
>  		 * The barrier synchronizes with the tlb_gen increment in
>  		 * the TLB shootdown code.
> +		 *
> +		 * As a future optimization opportunity, it's plausible
> +		 * that the x86 memory model is strong enough that this
> +		 * smp_mb() isn't needed.
>  		 */
>  		smp_mb();
>  		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
>  		if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
> -				next_tlb_gen)
> +		    next_tlb_gen) {
> +#ifdef CONFIG_MEMBARRIER
> +			/*
> +			 * We switched logical mm but we're not going to
> +			 * write to CR3.  We already did smp_mb() above,
> +			 * but membarrier() might require a sync_core()
> +			 * as well.
> +			 */
> +			if (unlikely(atomic_read(&next->membarrier_state) &
> +				     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE))
> +				sync_core_before_usermode();
> +#endif
> +
>  			return;
> +		}
>  
>  		/*
>  		 * TLB contents went out of date while we were in lazy
>  		 * mode. Fall through to the TLB switching code below.
> +		 * No need for an explicit membarrier invocation -- the CR3
> +		 * write will serialize.
>  		 */
>  		new_asid = prev_asid;
>  		need_flush = true;
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index e24b1fe348e3..24d97d1b6252 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -345,16 +345,6 @@ enum {
>  #include <asm/membarrier.h>
>  #endif
>  
> -static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> -{
> -	if (current->mm != mm)
> -		return;
> -	if (likely(!(atomic_read(&mm->membarrier_state) &
> -		     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
> -		return;
> -	sync_core_before_usermode();
> -}
> -
>  extern void membarrier_exec_mmap(struct mm_struct *mm);
>  
>  extern void membarrier_update_current_mm(struct mm_struct *next_mm);
> @@ -370,9 +360,6 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
>  static inline void membarrier_exec_mmap(struct mm_struct *mm)
>  {
>  }
> -static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> -{
> -}
>  static inline void membarrier_update_current_mm(struct mm_struct *next_mm)
>  {
>  }
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5226cc26a095..e4c122f8bf21 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4220,22 +4220,19 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>  	kmap_local_sched_in();
>  
>  	fire_sched_in_preempt_notifiers(current);
> +
>  	/*
>  	 * When switching through a kernel thread, the loop in
>  	 * membarrier_{private,global}_expedited() may have observed that
>  	 * kernel thread and not issued an IPI. It is therefore possible to
>  	 * schedule between user->kernel->user threads without passing though
>  	 * switch_mm(). Membarrier requires a barrier after storing to
> -	 * rq->curr, before returning to userspace, so provide them here:
> -	 *
> -	 * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly
> -	 *   provided by mmdrop(),
> -	 * - a sync_core for SYNC_CORE.
> +	 * rq->curr, before returning to userspace, and mmdrop() provides
> +	 * this barrier.
>  	 */
> -	if (mm) {
> -		membarrier_mm_sync_core_before_usermode(mm);
> +	if (mm)
>  		mmdrop(mm);
> -	}
> +
>  	if (unlikely(prev_state == TASK_DEAD)) {
>  		if (prev->sched_class->task_dead)
>  			prev->sched_class->task_dead(prev);
> -- 
> 2.31.1
> 
>