From: Peter Zijlstra <peterz@infradead•org>
To: Steven Rostedt <rostedt@goodmis•org>
Cc: mark.rutland@arm•com, catalin.marinas@arm•com,
linux-mm@kvack•org, guohanjun@huawei•com, will@kernel•org,
linux-arch@vger•kernel.org, yuzhao@google•com, maz@kernel•org,
suzuki.poulose@arm•com, steven.price@arm•com, arm@kernel•org,
Dave.Martin@arm•com, arnd@arndb•de,
Zhenyu Ye <yezhenyu2@huawei•com>,
npiggin@gmail•com, zhangshaokun@hisilicon•com,
broonie@kernel•org, xiexiangyou@huawei•com,
prime.zeng@hisilicon•com, kuhn.chenqun@huawei•com,
tglx@linutronix•de, linux-arm-kernel@lists•infradead.org,
linux-kernel@vger•kernel.org, aneesh.kumar@linux•ibm.com,
akpm@linux-foundation•org
Subject: Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range
Date: Tue, 21 Apr 2020 10:30:43 +0200 [thread overview]
Message-ID: <20200421083043.GP20730@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200420200616.44c7c7ea@oasis.local.home>
On Mon, Apr 20, 2020 at 08:06:16PM -0400, Steven Rostedt wrote:
> Peter Zijlstra <peterz@infradead•org> wrote:
> > On Fri, Apr 03, 2020 at 05:00:48PM +0800, Zhenyu Ye wrote:
> > > +static inline int tlb_get_level(struct mmu_gather *tlb)
> > > +{
> > > + int sum = tlb->cleared_ptes + tlb->cleared_pmds +
> > > + tlb->cleared_puds + tlb->cleared_p4ds;
> > > +
> > > + if (sum != 1)
> > > + return 0;
> > > + else if (tlb->cleared_ptes)
> > > + return 3;
> > > + else if (tlb->cleared_pmds)
> > > + return 2;
> > > + else if (tlb->cleared_puds)
> > > + return 1;
> > > +
> > > + return 0;
> > > +}
> >
> > That's some mighty wonky code. Please look at the generated asm.
>
> Without even looking at the generated asm, if a condition returns,
> there's no reason to add an else for that condition.
Not really the point; he wants to guarantee he only returns >0 when
there's a single bit set. But the thing is, cleared_* is a bitfield, and
I'm afraid that the above will result in some terrible code-gen.
Maybe something like:
if (tlb->cleared_ptes && !(tlb->cleared_pmds ||
tlb->cleared_puds ||
tlb->cleared_p4ds))
return 3;
if (tlb->cleared_pmds && !(tlb->cleared_ptes ||
tlb->cleared_puds ||
tlb->cleared_p4ds))
return 2;
if (tlb->cleared_puds && !(tlb->cleared_ptes ||
tlb->cleared_pmds ||
tlb->cleared_p4ds))
return 1;
return 0;
Which I admit is far too much typing, but I suspect it generates far
saner code (just a few masks and branches).
But maybe the compiler surprises us, what do I konw.
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next prev parent reply other threads:[~2020-04-21 8:31 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-03 9:00 [PATCH v1 0/6] arm64: tlb: add support for TTL feature Zhenyu Ye
2020-04-03 9:00 ` [PATCH v1 1/6] arm64: Detect the ARMv8.4 " Zhenyu Ye
2020-04-21 16:53 ` Christoph Hellwig
2020-04-21 17:13 ` Peter Zijlstra
2020-04-21 17:16 ` Christoph Hellwig
2020-04-22 2:13 ` Zhenyu Ye
2020-04-03 9:00 ` [PATCH v1 2/6] arm64: Add level-hinted TLB invalidation helper Zhenyu Ye
2020-04-03 9:00 ` [PATCH v1 3/6] arm64: Add tlbi_user_level " Zhenyu Ye
2020-04-03 9:00 ` [PATCH v1 4/6] tlb: mmu_gather: add tlb_set_*_range APIs Zhenyu Ye
2020-04-20 11:46 ` Peter Zijlstra
2020-04-03 9:00 ` [PATCH v1 5/6] mm: tlb: Provide flush_*_tlb_range wrappers Zhenyu Ye
2020-04-20 12:09 ` Peter Zijlstra
2020-04-21 14:18 ` Zhenyu Ye
2020-04-03 9:00 ` [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range Zhenyu Ye
2020-04-20 12:10 ` Peter Zijlstra
2020-04-21 0:06 ` Steven Rostedt
2020-04-21 8:30 ` Peter Zijlstra [this message]
2020-04-21 12:22 ` Zhenyu Ye
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