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References: <20200824052025.48362-1-benbjiang@tencent.com>
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From:   Jiang Biao <benbjiang@gmail.com>
Date:   Thu, 27 Aug 2020 17:49:12 +0800
Message-ID: <CAPJCdBmLeD84kRXWmuPj+-_2kBLmZ8wR-uJ641xShza6E52D0w@mail.gmail.com>
Subject: Re: [PATCH] driver/pci: reduce the single block time in pci_read_config
To:     Bjorn Helgaas <bhelgaas@google.com>
Cc:     linux-pci@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Jiang Biao <benbjiang@tencent.com>,
        Bin Lai <robinlai@tencent.com>
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Sender: linux-kernel-owner@vger.kernel.org
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kindly ping :)

On Mon, 24 Aug 2020 at 13:20, Jiang Biao <benbjiang@gmail.com> wrote:
>
> From: Jiang Biao <benbjiang@tencent.com>
>
> pci_read_config() could block several ms in kernel space, mainly
> caused by the while loop to call pci_user_read_config_dword().
> Singel pci_user_read_config_dword() loop could consume 130us+,
>               |    pci_user_read_config_dword() {
>               |      _raw_spin_lock_irq() {
> ! 136.698 us  |        native_queued_spin_lock_slowpath();
> ! 137.582 us  |      }
>               |      pci_read() {
>               |        raw_pci_read() {
>               |          pci_conf1_read() {
>   0.230 us    |            _raw_spin_lock_irqsave();
>   0.035 us    |            _raw_spin_unlock_irqrestore();
>   8.476 us    |          }
>   8.790 us    |        }
>   9.091 us    |      }
> ! 147.263 us  |    }
> and dozens of the loop could consume ms+.
>
> If we execute some lspci commands concurrently, ms+ scheduling
> latency could be detected.
>
> Add scheduling chance in the loop to improve the latency.
>
> Reported-by: Bin Lai <robinlai@tencent.com>
> Signed-off-by: Jiang Biao <benbjiang@tencent.com>
> ---
>  drivers/pci/pci-sysfs.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 6d78df9..3b9f63d 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -708,6 +708,7 @@ static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
>                 data[off - init_off + 3] = (val >> 24) & 0xff;
>                 off += 4;
>                 size -= 4;
> +               cond_resched();
>         }
>
>         if (size >= 2) {
> --
> 1.8.3.1
>