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

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Message-ID: <2975f6a4-58ef-4499-b177-4b73ead4c670@gmail.com>
Date: Mon, 20 Jan 2025 16:14:49 +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 20/01/2025 16:02, Rik van Riel wrote:
> On Mon, 2025-01-20 at 11:56 +0200, Nadav Amit wrote:
>>
>>> @@ -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).
> 
> It is clean, but I'm not convinced it is good enough.
> 
> We need to guarantee that the INVLPGBs have finished
> before we free the pages.
> 
> Running a TLBSYNC at the next context switch could
> mean that TLBSYNC won't run until after the pages
> have been freed.
> 
> In practice it is probably good enough, since it
> would be simpler for TLBSYNC to return once all
> pending (older) INVLPGBs have finished, but it's
> not architecturally guaranteed.
> 
> We could send an IPI to remote CPUs in order for
> them to call TLBSYNC, but is that really better?
> 

I am not sure we are on the same page. What I suggested is:

1. arch_tlbbatch_flush() would still do tlbsync()
2. No migrate_enable() in arch_tlbbatch_flush()
3. No migrate_disable() in arch_tlbbatch_add_pending()
4. arch_tlbbatch_add_pending() sets cpu_tlbstate.pending_tlb_broadcast
5. switch_mm_irqs_off() checks cpu_tlbstate.pending_tlb_broadcast and if 
it is set performs tlbsync and clears it.