Re: [PATCH RESEND v3 1/2] mm/tlb: skip redundant IPI when TLB flush already synchronized

[David Hildenbrand (Red Hat)]

>   
>   static void tlb_table_flush(struct mmu_gather *tlb)
> @@ -367,7 +378,7 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
>   		*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT);
>   		if (*batch == NULL) {
>   			tlb_table_invalidate(tlb);
> -			tlb_remove_table_one(table);
> +			tlb_remove_table_one(table, tlb);
>   			return;
>   		}
>   		(*batch)->nr = 0;
> @@ -427,6 +438,7 @@ static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
>   	tlb->vma_pfn = 0;
>   
>   	tlb->fully_unshared_tables = 0;
> +	tlb->tlb_flush_sent_ipi = 0;
>   	__tlb_reset_range(tlb);
>   	inc_tlb_flush_pending(tlb->mm);
>   }

But when would we have to reset tlb->tlb_flush_sent_ipi = 0 later? 
That's where it gets tricky. Just imagine the MMU gather gets reused later.

Also,

	+	if (info->freed_tables && info->tlb)
	+		info->tlb->tlb_flush_sent_ipi = true;

in native_flush_tlb_multi() misses the fact that we have different 
flushing types for removed/unshared tables vs. other flush.

So this approach more here certainly gets more complicated and error prone.

tlb_table_flush_implies_ipi_broadcast() was clearer in that regard: if 
you flushed the TLB after removing /unsharing tables, the IPI for 
handling page tables can be skipped. It's on the code flow to assure that.

What could work is tracking "tlb_table_flush_sent_ipi" really when we 
are flushing the TLB for removed/unshared tables, and maybe resetting it 
... I don't know when from the top of my head.

v2 was simpler IMHO.

--
Cheers

David