linux-kernel - Re: [PATCH 5/9] x86/mm/tlb: Optimize local TLB flushes

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Message-ID: <86e04985-7884-3d33-c479-92614b4e4342@intel.com>
Date:   Tue, 25 Jun 2019 14:36:44 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 5/9] x86/mm/tlb: Optimize local TLB flushes

On 6/12/19 11:48 PM, Nadav Amit wrote:
> While the updated smp infrastructure is capable of running a function on
> a single local core, it is not optimized for this case. 

OK, so flush_tlb_multi() is optimized for flushing local+remote at the
same time and is also (near?) the most optimal way to flush remote-only.
 But, it's not as optimized at doing local-only flushes.  But,
flush_tlb_on_cpus() *is* optimized for local-only flushes.

Right?

Can we distill that down to any particular advise that we can comment
these suckers with?  For instance, flush_tlb_multi() is apparently not
safe to call ever by itself without a 'flush_tlb_multi_enabled' check
first.  It's also, suboptimal for local flushes apparently.