linux-kernel - Re: [PATCH v5 10/12] x86,tlb: do targeted broadcast flushing from tlbbatch code

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Message-ID: <13bc0c49-09a4-434e-bd35-1ea50be38e25@gmail.com>
Date: Mon, 20 Jan 2025 11:56:29 +0200
From: Nadav Amit <nadav.amit@...il.com>
To: Rik van Riel <riel@...riel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, bp@...en8.de, peterz@...radead.org,
 dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com,
 thomas.lendacky@....com, kernel-team@...a.com, linux-mm@...ck.org,
 akpm@...ux-foundation.org, jannh@...gle.com, mhklinux@...look.com,
 andrew.cooper3@...rix.com
Subject: Re: [PATCH v5 10/12] x86,tlb: do targeted broadcast flushing from
 tlbbatch code



On 16/01/2025 4:30, Rik van Riel wrote:
> Instead of doing a system-wide TLB flush from arch_tlbbatch_flush,
> queue up asynchronous, targeted flushes from arch_tlbbatch_add_pending.
> 

[snip]

> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -1659,9 +1659,7 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
>   	 * a local TLB flush is needed. Optimize this use-case by calling
>   	 * flush_tlb_func_local() directly in this case.
>   	 */
> -	if (cpu_feature_enabled(X86_FEATURE_INVLPGB)) {
> -		invlpgb_flush_all_nonglobals();
> -	} else if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids) {
> +	if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids) {
>   		flush_tlb_multi(&batch->cpumask, info);
>   	} else if (cpumask_test_cpu(cpu, &batch->cpumask)) {
>   		lockdep_assert_irqs_enabled();
> @@ -1670,12 +1668,62 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
>   		local_irq_enable();
>   	}
>   
> +	/*
> +	 * If we issued (asynchronous) INVLPGB flushes, wait for them here.
> +	 * The cpumask above contains only CPUs that were running tasks
> +	 * not using broadcast TLB flushing.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_INVLPGB) && batch->used_invlpgb) {
> +		tlbsync();
> +		migrate_enable();

Maybe someone mentioned it before, but I would emphasize that I do not 
think that preventing migration for potentially long time is that great.

One alternative solution would be to set a bit on cpu_tlbstate, that 
when set, you'd issue a tlbsync on context switch.

(I can think about other solutions, but I think the one I just mentioned 
is the cleanest one).

> +		batch->used_invlpgb = false;
> +	}
> +
>   	cpumask_clear(&batch->cpumask);
>   
>   	put_flush_tlb_info();
>   	put_cpu();
>   }
>   
> +void arch_tlbbatch_add_pending(struct arch_tlbflush_unmap_batch *batch,
> +					     struct mm_struct *mm,
> +					     unsigned long uaddr)
> +{
> +	if (static_cpu_has(X86_FEATURE_INVLPGB) && mm_global_asid(mm)) {
> +		u16 asid = mm_global_asid(mm);
> +		/*
> +		 * Queue up an asynchronous invalidation. The corresponding
> +		 * TLBSYNC is done in arch_tlbbatch_flush(), and must be done
> +		 * on the same CPU.
> +		 */
> +		if (!batch->used_invlpgb) {
> +			batch->used_invlpgb = true;
> +			migrate_disable();

See my comment above...

> +		}
> +		invlpgb_flush_user_nr_nosync(kern_pcid(asid), uaddr, 1, false);
> +		/* Do any CPUs supporting INVLPGB need PTI? */
> +		if (static_cpu_has(X86_FEATURE_PTI))
> +			invlpgb_flush_user_nr_nosync(user_pcid(asid), uaddr, 1, false);
> +
> +		/*
> +		 * Some CPUs might still be using a local ASID for this
> +		 * process, and require IPIs, while others are using the
> +		 * global ASID.
> +		 *
> +		 * In this corner case we need to do both the broadcast
> +		 * TLB invalidation, and send IPIs. The IPIs will help
> +		 * stragglers transition to the broadcast ASID.
> +		 */
> +		if (READ_ONCE(mm->context.asid_transition))
> +			goto also_send_ipi;
> +	} else {
> +also_send_ipi:

I really think you should avoid such goto's. A simple bool variable of 
"need_ipi" would suffice.

> +		inc_mm_tlb_gen(mm);
> +		cpumask_or(&batch->cpumask, &batch->cpumask, mm_cpumask(mm));
> +	}
> +	mmu_notifier_arch_invalidate_secondary_tlbs(mm, 0, -1UL);
> +}
> +
>   /*
>    * Blindly accessing user memory from NMI context can be dangerous
>    * if we're in the middle of switching the current user task or