From: "H. Peter Anvin" <hpa@zytor.com>
To: Andi Kleen <andi@firstfloor.org>, bhelgaas@google.com
Cc: 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 10:28:17 -0700 [thread overview]
Message-ID: <0bb90a87-e03d-2b14-0e58-bd0f41bf84a9@zytor.com> (raw)
In-Reply-To: <20170302232104.10136-1-andi@firstfloor.org>
On 03/02/17 15:21, 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.
>
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
> arch/x86/pci/mmconfig_64.c | 1 +
> drivers/pci/access.c | 14 ++++++++++----
> include/linux/pci.h | 2 ++
> 3 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/pci/mmconfig_64.c b/arch/x86/pci/mmconfig_64.c
> index bea52496aea6..8bf10f41e626 100644
> --- a/arch/x86/pci/mmconfig_64.c
> +++ b/arch/x86/pci/mmconfig_64.c
> @@ -121,6 +121,7 @@ int __init pci_mmcfg_arch_init(void)
> }
>
> raw_pci_ext_ops = &pci_mmcfg;
> + pci_root_ops.ll_allowed = true;
>
"ll_allowed" is pretty awful naming... you spend almost all the
characters telling us nothing. I spend several seconds trying to figure
out what "ll" stood for, and without the context of the patch I'd have
had to go a massive grep. Just call it "lockless" or something.
-hpa
prev parent reply other threads:[~2017-03-14 17:29 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 ` [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig Thomas Gleixner
2017-03-14 17:28 ` H. Peter Anvin [this message]
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