Re: [PATCH v3] PCI: Speed up device init by parsing capabilities all at once

[Ajay Kaher <akaher@vmware.com>]

On 23/02/22, 5:20 PM, "Ajay Kaher" <akaher@vmware.com> wrote:

> From: Vikash Bansal <bvikas@vmware.com>

Bjorn, as per your guidance we sent v3 (hope it's as per your expectation). 

Please let us know if you are looking for more improvement or 
wanted us to re-sent this patch.

-Ajay

> In the current implementation, the PCI capability list is parsed from
> the beginning to find each capability, which results in a large number
> of redundant PCI reads.
> 
> Instead, we can parse the complete list just once, store it in the
> pci_dev structure, and get the offset of each capability directly from
> the pci_dev structure.
> 
> This implementation improves pci devices initialization time  by ~2-3%
> (from 270ms to 261ms) in case of bare metal and 7-8% (From 201ms to 184ms)
> in case of VM running on ESXi.
> 
> It also adds a memory overhead of 20bytes (value of PCI_CAP_ID_MAX) per
> PCI device.
> 
> Ran pahole for pci_dev structure. This patch is not adding any padding
> bytes.
> 
> Signed-off-by: Vikash Bansal <bvikas@vmware.com>
> Signed-off-by: Ajay Kaher <akaher@vmware.com>
> 
> ---
> 
> Changes in v3:
>   - Added check before access cap_off from pci_find_capability().
>   - Moved declaration of pci_find_all_capabilities() to drivers/pci/pci.h.
>
> Changes in v2:
>  - Ran pahole tool.
>  - Modified comments to add "clock time".
>  - Removed comments before call to pci_find_all_capabilities.
>
> ---
> drivers/pci/pci.c   | 44 +++++++++++++++++++++++++++++++++++++-------
> drivers/pci/pci.h   |  1 +
> drivers/pci/probe.c |  1 +
> include/linux/pci.h |  1 +
> 4 files changed, 40 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 3d2fb39..cf54811 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -469,6 +469,40 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
> }
>
> /**
> + * pci_find_all_capabilities - Read all capabilities
> + * @dev: the PCI device
> + *
> + * Read all capabilities and store offsets in cap_off
> + * array in pci_dev structure.
> + */
> +void pci_find_all_capabilities(struct pci_dev *dev)
> +{
> +	int ttl = PCI_FIND_CAP_TTL;
> +	u16 ent;
> +	u8 pos;
> +	u8 id;
> +
> +	pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
> +	if (!pos)
> +		return;
> +	pci_bus_read_config_byte(dev->bus, dev->devfn, pos, &pos);
> +	while (ttl--) {
> +		if (pos < 0x40)
> +			break;
> +		pos &= ~3;
> +		pci_bus_read_config_word(dev->bus, dev->devfn, pos, &ent);
> +		id = ent & 0xff;
> +		if (id == 0xff)
> +			break;
> +
> +		/* Read first instance of capability */
> +		if (!(dev->cap_off[id]))
> +			dev->cap_off[id] = pos;
> +		pos = (ent >> 8);
> +	}
> +}
> +
> +/**
>   * pci_find_capability - query for devices' capabilities
>   * @dev: PCI device to query
>   * @cap: capability code
> @@ -489,13 +523,9 @@ static u8 __pci_bus_find_cap_start(struct pci_bus *bus,
>   */
> u8 pci_find_capability(struct pci_dev *dev, int cap)
> {
> -	u8 pos;
> -
> -	pos = __pci_bus_find_cap_start(dev->bus, dev->devfn, dev->hdr_type);
> -	if (pos)
> -		pos = __pci_find_next_cap(dev->bus, dev->devfn, pos, cap);
> -
> -	return pos;
> +	if(cap >= PCI_CAP_ID_MAX)
> +		return 0;
> +	return dev->cap_off[cap];
> }
> EXPORT_SYMBOL(pci_find_capability);
> 
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 3d60cab..3cb70d5 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -87,6 +87,7 @@ bool pci_bridge_d3_possible(struct pci_dev *dev);
> void pci_bridge_d3_update(struct pci_dev *dev);
> void pci_bridge_wait_for_secondary_bus(struct pci_dev *dev);
> void pci_bridge_reconfigure_ltr(struct pci_dev *dev);
> +void pci_find_all_capabilities(struct pci_dev *dev);
>
> static inline void pci_wakeup_event(struct pci_dev *dev)
> {
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 087d365..d75e1fa 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1839,6 +1839,7 @@ int pci_setup_device(struct pci_dev *dev)
> 	dev->hdr_type = hdr_type & 0x7f;
> 	dev->multifunction = !!(hdr_type & 0x80);
> 	dev->error_state = pci_channel_io_normal;
> +	pci_find_all_capabilities(dev);
> 	set_pcie_port_type(dev);
>
> 	pci_set_of_node(dev);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 18a75c8..bd61cee 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -326,6 +326,7 @@ struct pci_dev {
> 	unsigned int	class;		/* 3 bytes: (base,sub,prog-if) */
> 	u8		revision;	/* PCI revision, low byte of class word */
> 	u8		hdr_type;	/* PCI header type (`multi' flag masked out) */
> +	u8		cap_off[PCI_CAP_ID_MAX]; /* Offsets of all pci capabilities */
> #ifdef CONFIG_PCIEAER
> 	u16		aer_cap;	/* AER capability offset */
> 	struct aer_stats *aer_stats;	/* AER stats for this device */
> --
> 2.7.4