Re: [PATCH] parisc: Update huge TLB page support to use per-pagetable spinlock

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sat, 2019-04-27 at 14:15 -0400, John David Anglin wrote:
> This patch updates the parisc huge TLB page support to use per-
> pagetable spinlocks.
> 
> This patch requires Mikulas' per-pagetable spinlock patch and the
> revised TLB serialization patch from Helge and myself.  With Mikulas'
> patch, we need to use the per-pagetable spinlock for page table
> updates.  The TLB lock is only used to serialize TLB flushes on
> machines with the Merced bus.
> 
> Signed-off-by: John David Anglin <dave.anglin@bell.net>
> ---
> 
> diff --git a/arch/parisc/mm/hugetlbpage.c
> b/arch/parisc/mm/hugetlbpage.c
> index d77479ae3af2..d578809e55cf 100644
> --- a/arch/parisc/mm/hugetlbpage.c
> +++ b/arch/parisc/mm/hugetlbpage.c
> @@ -139,9 +139,9 @@ void set_huge_pte_at(struct mm_struct *mm,
> unsigned long addr,
>  {
>  	unsigned long flags;
> 
> -	purge_tlb_start(flags);
> +	spin_lock_irqsave(pgd_spinlock((mm)->pgd), flags);
>  	__set_huge_pte_at(mm, addr, ptep, entry);
> -	purge_tlb_end(flags);
> +	spin_unlock_irqrestore(pgd_spinlock((mm)->pgd), flags);

You can't do stuff like this on the problem systems: we can only have
one outstanding purge at once on the inter-cpu bus.  For them,
therefore, we also need a global spinlock.

Pulling the tlb purge lock down to the lowest level, so inside
purge_tlb_entries() might address a lot of the performance issues while
keeping the purges correct on merced class.

James