linux-kernel - Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb

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Date:   Wed, 21 Jun 2017 10:32:13 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the
 flushing algorithm

On Tue, 20 Jun 2017, Andy Lutomirski wrote:
>  struct flush_tlb_info {
> +	/*
> +	 * We support several kinds of flushes.
> +	 *
> +	 * - Fully flush a single mm.  flush_mm will be set, flush_end will be

flush_mm is the *mm member in the struct, right? You might rename that as a
preparatory step so comments and implementation match.

> +	 *   TLB_FLUSH_ALL, and new_tlb_gen will be the tlb_gen to which the
> +	 *   IPI sender is trying to catch us up.
> +	 *
> +	 * - Partially flush a single mm.  flush_mm will be set, flush_start
> +	 *   and flush_end will indicate the range, and new_tlb_gen will be
> +	 *   set such that the changes between generation new_tlb_gen-1 and
> +	 *   new_tlb_gen are entirely contained in the indicated range.
> +	 *
> +	 * - Fully flush all mms whose tlb_gens have been updated.  flush_mm
> +	 *   will be NULL, flush_end will be TLB_FLUSH_ALL, and new_tlb_gen
> +	 *   will be zero.
> +	 */
>  	struct mm_struct *mm;
>  	unsigned long start;
>  	unsigned long end;
> +	u64 new_tlb_gen;

Nit. While at it could you please make that struct tabular aligned as we
usually do in x86?

>  static void flush_tlb_func_common(const struct flush_tlb_info *f,
>  				  bool local, enum tlb_flush_reason reason)
>  {
> +	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
> +
> +	/*
> +	 * Our memory ordering requirement is that any TLB fills that
> +	 * happen after we flush the TLB are ordered after we read
> +	 * active_mm's tlb_gen.  We don't need any explicit barrier
> +	 * because all x86 flush operations are serializing and the
> +	 * atomic64_read operation won't be reordered by the compiler.
> +	 */

Can you please move the comment above the loaded_mm assignment? 

> +	u64 mm_tlb_gen = atomic64_read(&loaded_mm->context.tlb_gen);
> +	u64 local_tlb_gen = this_cpu_read(cpu_tlbstate.ctxs[0].tlb_gen);
> +
>  	/* This code cannot presently handle being reentered. */
>  	VM_WARN_ON(!irqs_disabled());
>  
> +	VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[0].ctx_id) !=
> +		   loaded_mm->context.ctx_id);
> +
>  	if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
> +		/*
> +		 * leave_mm() is adequate to handle any type of flush, and
> +		 * we would prefer not to receive further IPIs.

While I know what you mean, it might be useful to have a more elaborate
explanation why this prevents new IPIs.

> +		 */
>  		leave_mm(smp_processor_id());
>  		return;
>  	}
>  
> -	if (f->end == TLB_FLUSH_ALL) {
> -		local_flush_tlb();
> -		if (local)
> -			count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
> -		trace_tlb_flush(reason, TLB_FLUSH_ALL);
> -	} else {
> +	if (local_tlb_gen == mm_tlb_gen) {
> +		/*
> +		 * There's nothing to do: we're already up to date.  This can
> +		 * happen if two concurrent flushes happen -- the first IPI to
> +		 * be handled can catch us all the way up, leaving no work for
> +		 * the second IPI to be handled.

That not restricted to IPIs, right? A local flush / IPI combo can do that
as well.

Other than those nits;

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>