From: Thomas Gleixner <tglx@linutronix.de>
To: Andi Kleen <andi@firstfloor.org>
Cc: bhelgaas@google.com, x86@kernel.org, linux-pci@vger.kernel.org,
eranian@google.com, peterz@infradead.org,
linux-kernel@vger.kernel.org, Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig
Date: Tue, 14 Mar 2017 14:06:13 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.20.1703141106231.3619@nanos> (raw)
In-Reply-To: <20170302232104.10136-1-andi@firstfloor.org>
On Thu, 2 Mar 2017, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> The Intel uncore driver can do a lot of PCI config accesses to read
> performance counters. I had a situation on a 4S system where it
> was spending 40+% of CPU time grabbing the pci_cfg_lock due to that.
>
> For 64bit x86 with MMCONFIG there isn't really any reason to take
> a lock. The access is directly mapped to an underlying MMIO area,
> which can fully operate lockless.
>
> Add a new flag that allows the PCI mid layer to skip the lock
> and set it for the 64bit mmconfig code.
>
> There's a small risk that someone relies on this lock for synchronization,
> but I think that's unlikely because there isn't really any useful
> synchronization at this individual operation level. Any useful
> synchronization would likely need to protect at least a
> read-modify-write or similar. So I made it unconditional without opt-in.
This part of the changelog is just crap.
The reason why pci_lock exists and is taken for each single read/write
config is that some ops implementations, e.g. the generic ones, must
protect at this granularity level because
ops->map_bus()
read/writeX()
needs to be 'atomic'.
MMCONFIG obviously does not require this at all because it's a simple
byte/word/dword read/write which is serialized by itself. So it's obvious
that the serialization with pci_lock is pointless in this case.
It's not that hard to figure it out and write up a proper changelog instead
of handwaving about risk and whatever.
Thanks,
tglx
next prev parent reply other threads:[~2017-03-14 13:06 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-02 23:21 [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig Andi Kleen
2017-03-02 23:21 ` [PATCH 2/4] pci: Add generic pci_bus_force_mmconfig interface Andi Kleen
2017-03-14 17:34 ` H. Peter Anvin
2017-03-02 23:21 ` [PATCH 3/4] x86, pci: Add interface to force mmconfig Andi Kleen
2017-03-14 13:55 ` Thomas Gleixner
2017-03-14 15:41 ` Andi Kleen
2017-03-14 16:40 ` Thomas Gleixner
2017-03-14 17:02 ` Andi Kleen
2017-03-14 17:56 ` Thomas Gleixner
2017-03-14 19:47 ` Bjorn Helgaas
2017-03-15 2:24 ` Andi Kleen
2017-03-15 2:55 ` Bjorn Helgaas
2017-03-15 10:00 ` Thomas Gleixner
2017-03-15 14:09 ` Bjorn Helgaas
2017-03-16 0:02 ` Andi Kleen
2017-03-16 22:45 ` Thomas Gleixner
2017-03-02 23:21 ` [PATCH 4/4] perf/x86/intel/uncore: Enable forced mmconfig for Intel uncore Andi Kleen
2017-03-14 13:06 ` Thomas Gleixner [this message]
2017-03-14 17:28 ` [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig H. Peter Anvin
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