| Message ID | 20180903180242.14504-2-mr.nuke.me@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Delegated to: | Bjorn Helgaas |
| Headers | show |
| Series | Export PCIe bandwidth via sysfs | expand |
On Mon, 3 Sep 2018 13:02:28 -0500 Alexandru Gagniuc <mr.nuke.me@gmail.com> wrote: > For certain bandwidth-critical devices (e.g. multi-port network cards) > it is useful to know the available bandwidth to the root complex. This > information is only available via the system log, which doesn't > account for link degradation after probing. > > With a sysfs attribute, we can computes the bandwidth on-demand, and > will detect degraded links. > > Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> In other places (like /sys/class/net/eth0/speed) only the raw value is printed without suffix. The general convention in sysfs is that it should be one value per file and in more raw format. So why not just print it in bits/sec without suffix?
On 09/05/2018 02:26 AM, Stephen Hemminger wrote: > On Mon, 3 Sep 2018 13:02:28 -0500 > Alexandru Gagniuc <mr.nuke.me@gmail.com> wrote: > >> For certain bandwidth-critical devices (e.g. multi-port network cards) >> it is useful to know the available bandwidth to the root complex. This >> information is only available via the system log, which doesn't >> account for link degradation after probing. >> >> With a sysfs attribute, we can computes the bandwidth on-demand, and >> will detect degraded links. >> >> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> > > In other places (like /sys/class/net/eth0/speed) only the raw value is printed > without suffix. The general convention in sysfs is that it should be one value > per file and in more raw format. So why not just print it in bits/sec without > suffix? I wanted to be consistent with other PCIe exports that use units.For example: /sys/devices/pci0000:3a/0000:3a:00.0/0000:3b:00.0/0000:3c:05.0/max_link_speed:8 GT/s /sys/devices/pci0000:3a/0000:3a:00.0/0000:3b:00.0/0000:3c:05.0/current_link_speed:2.5 GT/s /sys/bus/pci/slots/182/cur_bus_speed:2.5 GT/s PCIe /sys/bus/pci/slots/182/max_bus_speed:8.0 GT/s PCIe
[+cc Stephen, Martin (for possible lspci changes)] Hi Alexandru, On Mon, Sep 03, 2018 at 01:02:28PM -0500, Alexandru Gagniuc wrote: > For certain bandwidth-critical devices (e.g. multi-port network cards) > it is useful to know the available bandwidth to the root complex. This > information is only available via the system log, which doesn't > account for link degradation after probing. > > With a sysfs attribute, we can computes the bandwidth on-demand, and > will detect degraded links. > > Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> > --- > drivers/pci/pci-sysfs.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c > index 9ecfe13157c0..6658e927b1f5 100644 > --- a/drivers/pci/pci-sysfs.c > +++ b/drivers/pci/pci-sysfs.c > @@ -218,6 +218,18 @@ static ssize_t current_link_width_show(struct device *dev, > } > static DEVICE_ATTR_RO(current_link_width); > > +static ssize_t available_bandwidth_show(struct device *dev, > + struct device_attribute *attr, char *buf) > +{ > + struct pci_dev *pci_dev = to_pci_dev(dev); > + u32 bw_avail; > + > + bw_avail = pcie_bandwidth_available(pci_dev, NULL, NULL, NULL); > + > + return sprintf(buf, "%u.%03u Gb/s\n", bw_avail / 1000, bw_avail % 1000); > +} > +static DEVICE_ATTR_RO(available_bandwidth); Help me understand this. We already have these sysfs attributes: max_link_speed # eg, 16 GT/s max_link_width # eg, 8 current_link_speed # eg, 16 GT/s current_link_width # eg, 8 so I think the raw materials are already exposed. The benefits I see for this new file are that - pcie_bandwidth_available() does the work of traversing up the tree, doing the computations (link width * speed, reduced by encoding overhead), and finding the minimum, and - it re-traverses the path every time we look at it, while the boot-time check is a one-time event. In principle this could all be done in user space with the attributes that are already exported. There's some precedent for things like this in lspci, e.g., "NUMA node" [1], and lspci might even be a more user-friendly place for users to look for this, as opposed to searching through sysfs. [1] https://git.kernel.org/pub/scm/utils/pciutils/pciutils.git/commit/?id=90ec4a6d0ae8 > static ssize_t secondary_bus_number_show(struct device *dev, > struct device_attribute *attr, > char *buf) > @@ -786,6 +798,7 @@ static struct attribute *pcie_dev_attrs[] = { > &dev_attr_current_link_width.attr, > &dev_attr_max_link_width.attr, > &dev_attr_max_link_speed.attr, > + &dev_attr_available_bandwidth.attr, > NULL, > }; > > -- > 2.17.1 >
On 10/03/2018 04:31 PM, Bjorn Helgaas wrote: > > [EXTERNAL EMAIL] > Please report any suspicious attachments, links, or requests for sensitive information. > > > [+cc Stephen, Martin (for possible lspci changes)] > > Hi Alexandru, > > On Mon, Sep 03, 2018 at 01:02:28PM -0500, Alexandru Gagniuc wrote: >> For certain bandwidth-critical devices (e.g. multi-port network cards) >> it is useful to know the available bandwidth to the root complex. This >> information is only available via the system log, which doesn't >> account for link degradation after probing. >> >> With a sysfs attribute, we can computes the bandwidth on-demand, and >> will detect degraded links. >> >> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> >> --- >> drivers/pci/pci-sysfs.c | 13 +++++++++++++ >> 1 file changed, 13 insertions(+) >> >> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c >> index 9ecfe13157c0..6658e927b1f5 100644 >> --- a/drivers/pci/pci-sysfs.c >> +++ b/drivers/pci/pci-sysfs.c >> @@ -218,6 +218,18 @@ static ssize_t current_link_width_show(struct device *dev, >> } >> static DEVICE_ATTR_RO(current_link_width); >> >> +static ssize_t available_bandwidth_show(struct device *dev, >> + struct device_attribute *attr, char *buf) >> +{ >> + struct pci_dev *pci_dev = to_pci_dev(dev); >> + u32 bw_avail; >> + >> + bw_avail = pcie_bandwidth_available(pci_dev, NULL, NULL, NULL); >> + >> + return sprintf(buf, "%u.%03u Gb/s\n", bw_avail / 1000, bw_avail % 1000); >> +} >> +static DEVICE_ATTR_RO(available_bandwidth); > > Help me understand this. We already have these sysfs attributes: > > max_link_speed # eg, 16 GT/s > max_link_width # eg, 8 > current_link_speed # eg, 16 GT/s > current_link_width # eg, 8 > > so I think the raw materials are already exposed. > > The benefits I see for this new file are that > > - pcie_bandwidth_available() does the work of traversing up the > tree, doing the computations (link width * speed, reduced by > encoding overhead), and finding the minimum, and > > - it re-traverses the path every time we look at it, while the > boot-time check is a one-time event. > > In principle this could all be done in user space with the attributes > that are already exported. There's some precedent for things like > this in lspci, e.g., "NUMA node" [1], and lspci might even be a more > user-friendly place for users to look for this, as opposed to > searching through sysfs. Parsing the endpoint to root port bandwidth is, in principle, possible from userspace. It's just that in practice it's very clumsy to do, and, as you pointed out, not that reliable. I understand it's not information that all users would jump in the air to know. However, it was important enough for certain use cases, that the kernel already has a very reliable way to calculate it. It seems to me that the most elegant way is to let the kernel tell us, since the kernel already has this facility. To quote one of the texts under Documentation/, it is an elegant way to "avoid reinventing kernel wheels in userspace". Alex > [1] https://git.kernel.org/pub/scm/utils/pciutils/pciutils.git/commit/?id=90ec4a6d0ae8 > >> static ssize_t secondary_bus_number_show(struct device *dev, >> struct device_attribute *attr, >> char *buf) >> @@ -786,6 +798,7 @@ static struct attribute *pcie_dev_attrs[] = { >> &dev_attr_current_link_width.attr, >> &dev_attr_max_link_width.attr, >> &dev_attr_max_link_speed.attr, >> + &dev_attr_available_bandwidth.attr, >> NULL, >> }; >> >> -- >> 2.17.1 >> >
[+cc Alex (VC mentioned below)] On Wed, Oct 03, 2018 at 10:00:19PM +0000, Alex_Gagniuc@Dellteam.com wrote: > On 10/03/2018 04:31 PM, Bjorn Helgaas wrote: > > On Mon, Sep 03, 2018 at 01:02:28PM -0500, Alexandru Gagniuc wrote: > >> For certain bandwidth-critical devices (e.g. multi-port network cards) > >> it is useful to know the available bandwidth to the root complex. This > >> information is only available via the system log, which doesn't > >> account for link degradation after probing. > >> > >> With a sysfs attribute, we can computes the bandwidth on-demand, and > >> will detect degraded links. > >> > >> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> > >> --- > >> drivers/pci/pci-sysfs.c | 13 +++++++++++++ > >> 1 file changed, 13 insertions(+) > >> > >> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c > >> index 9ecfe13157c0..6658e927b1f5 100644 > >> --- a/drivers/pci/pci-sysfs.c > >> +++ b/drivers/pci/pci-sysfs.c > >> @@ -218,6 +218,18 @@ static ssize_t current_link_width_show(struct device *dev, > >> } > >> static DEVICE_ATTR_RO(current_link_width); > >> > >> +static ssize_t available_bandwidth_show(struct device *dev, > >> + struct device_attribute *attr, char *buf) > >> +{ > >> + struct pci_dev *pci_dev = to_pci_dev(dev); > >> + u32 bw_avail; > >> + > >> + bw_avail = pcie_bandwidth_available(pci_dev, NULL, NULL, NULL); > >> + > >> + return sprintf(buf, "%u.%03u Gb/s\n", bw_avail / 1000, bw_avail % 1000); > >> +} > >> +static DEVICE_ATTR_RO(available_bandwidth); > > > > Help me understand this. We already have these sysfs attributes: > > > > max_link_speed # eg, 16 GT/s > > max_link_width # eg, 8 > > current_link_speed # eg, 16 GT/s > > current_link_width # eg, 8 > > > > so I think the raw materials are already exposed. > > > > The benefits I see for this new file are that > > > > - pcie_bandwidth_available() does the work of traversing up the > > tree, doing the computations (link width * speed, reduced by > > encoding overhead), and finding the minimum, and > > > > - it re-traverses the path every time we look at it, while the > > boot-time check is a one-time event. > > > > In principle this could all be done in user space with the attributes > > that are already exported. There's some precedent for things like > > this in lspci, e.g., "NUMA node" [1], and lspci might even be a more > > user-friendly place for users to look for this, as opposed to > > searching through sysfs. > > Parsing the endpoint to root port bandwidth is, in principle, possible > from userspace. It's just that in practice it's very clumsy to do, and, > as you pointed out, not that reliable. I don't remember the reliability issue. Was that in a previous conversation? AFAICT, using current_link_speed and current_link_width should be as reliable as pcie_bandwidth_available() because they're both based on reading PCI_EXP_LNKSTA. This patch exposes only the available bandwidth, not the limiting device, link speed, or link width. Maybe that extra information isn't important in this context. Of course, it's easy to derive using current_link_speed and current_link_width, but if we do that, there's really no benefit to adding a new file. Linux doesn't really support Virtual Channels (VC) yet, and I don't know much about it, but it seems like Quality-of-Service features like VC could affect this idea of available bandwidth. Since we already expose the necessary information, and we might throw additional things like VC into the mix, I'm a little hesitant about adding things to sysfs because they're very difficult to change later. > I understand it's not information that all users would jump in the air > to know. However, it was important enough for certain use cases, that > the kernel already has a very reliable way to calculate it. > > It seems to me that the most elegant way is to let the kernel tell us, > since the kernel already has this facility. To quote one of the texts > under Documentation/, it is an elegant way to "avoid reinventing kernel > wheels in userspace". > > Alex > > > [1] https://git.kernel.org/pub/scm/utils/pciutils/pciutils.git/commit/?id=90ec4a6d0ae8 > > > >> static ssize_t secondary_bus_number_show(struct device *dev, > >> struct device_attribute *attr, > >> char *buf) > >> @@ -786,6 +798,7 @@ static struct attribute *pcie_dev_attrs[] = { > >> &dev_attr_current_link_width.attr, > >> &dev_attr_max_link_width.attr, > >> &dev_attr_max_link_speed.attr, > >> + &dev_attr_available_bandwidth.attr, > >> NULL, > >> }; > >> > >> -- > >> 2.17.1 > >> > > >
On 10/04/2018 03:45 PM, Bjorn Helgaas wrote: > > [EXTERNAL EMAIL] > Please report any suspicious attachments, links, or requests for sensitive information. > > > [+cc Alex (VC mentioned below)] > > On Wed, Oct 03, 2018 at 10:00:19PM +0000, Alex_Gagniuc@Dellteam.com wrote: >> On 10/03/2018 04:31 PM, Bjorn Helgaas wrote: >>> On Mon, Sep 03, 2018 at 01:02:28PM -0500, Alexandru Gagniuc wrote: >>>> For certain bandwidth-critical devices (e.g. multi-port network cards) >>>> it is useful to know the available bandwidth to the root complex. This >>>> information is only available via the system log, which doesn't >>>> account for link degradation after probing. >>>> >>>> With a sysfs attribute, we can computes the bandwidth on-demand, and >>>> will detect degraded links. >>>> >>>> Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> >>>> --- >>>> drivers/pci/pci-sysfs.c | 13 +++++++++++++ >>>> 1 file changed, 13 insertions(+) >>>> >>>> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c >>>> index 9ecfe13157c0..6658e927b1f5 100644 >>>> --- a/drivers/pci/pci-sysfs.c >>>> +++ b/drivers/pci/pci-sysfs.c >>>> @@ -218,6 +218,18 @@ static ssize_t current_link_width_show(struct device *dev, >>>> } >>>> static DEVICE_ATTR_RO(current_link_width); >>>> >>>> +static ssize_t available_bandwidth_show(struct device *dev, >>>> + struct device_attribute *attr, char *buf) >>>> +{ >>>> + struct pci_dev *pci_dev = to_pci_dev(dev); >>>> + u32 bw_avail; >>>> + >>>> + bw_avail = pcie_bandwidth_available(pci_dev, NULL, NULL, NULL); >>>> + >>>> + return sprintf(buf, "%u.%03u Gb/s\n", bw_avail / 1000, bw_avail % 1000); >>>> +} >>>> +static DEVICE_ATTR_RO(available_bandwidth); >>> >>> Help me understand this. We already have these sysfs attributes: >>> >>> max_link_speed # eg, 16 GT/s >>> max_link_width # eg, 8 >>> current_link_speed # eg, 16 GT/s >>> current_link_width # eg, 8 >>> >>> so I think the raw materials are already exposed. >>> >>> The benefits I see for this new file are that >>> >>> - pcie_bandwidth_available() does the work of traversing up the >>> tree, doing the computations (link width * speed, reduced by >>> encoding overhead), and finding the minimum, and >>> >>> - it re-traverses the path every time we look at it, while the >>> boot-time check is a one-time event. >>> >>> In principle this could all be done in user space with the attributes >>> that are already exported. There's some precedent for things like >>> this in lspci, e.g., "NUMA node" [1], and lspci might even be a more >>> user-friendly place for users to look for this, as opposed to >>> searching through sysfs. >> >> Parsing the endpoint to root port bandwidth is, in principle, possible >> from userspace. It's just that in practice it's very clumsy to do, and, >> as you pointed out, not that reliable. > > I don't remember the reliability issue. Was that in a previous > conversation? AFAICT, using current_link_speed and current_link_width > should be as reliable as pcie_bandwidth_available() because they're > both based on reading PCI_EXP_LNKSTA. > > This patch exposes only the available bandwidth, not the limiting > device, link speed, or link width. Maybe that extra information isn't > important in this context. Of course, it's easy to derive using > current_link_speed and current_link_width, but if we do that, there's > really no benefit to adding a new file. > > Linux doesn't really support Virtual Channels (VC) yet, and I don't > know much about it, but it seems like Quality-of-Service features like > VC could affect this idea of available bandwidth. > > Since we already expose the necessary information, and we might throw > additional things like VC into the mix, I'm a little hesitant about > adding things to sysfs because they're very difficult to change later. I understand your concerns with VC and crazy PCIe outliers. 'available_bandwidth' is meant to mean physical bandwidth, rather than "what comcast gives you". I see now the possibility for confusion. My motivation is to save drivers from the hassle of having to call pcie_print_link_status(), and prevent the rare duplicate syslog messages. I prefer re-using the kernel logic over doing it over again from userspace, but I won't insist over your doubts. Alex >> I understand it's not information that all users would jump in the air >> to know. However, it was important enough for certain use cases, that >> the kernel already has a very reliable way to calculate it. >> >> It seems to me that the most elegant way is to let the kernel tell us, >> since the kernel already has this facility. To quote one of the texts >> under Documentation/, it is an elegant way to "avoid reinventing kernel >> wheels in userspace". >> >> Alex >> >>> [1] https://git.kernel.org/pub/scm/utils/pciutils/pciutils.git/commit/?id=90ec4a6d0ae8 >>> >>>> static ssize_t secondary_bus_number_show(struct device *dev, >>>> struct device_attribute *attr, >>>> char *buf) >>>> @@ -786,6 +798,7 @@ static struct attribute *pcie_dev_attrs[] = { >>>> &dev_attr_current_link_width.attr, >>>> &dev_attr_max_link_width.attr, >>>> &dev_attr_max_link_speed.attr, >>>> + &dev_attr_available_bandwidth.attr, >>>> NULL, >>>> }; >>>> >>>> -- >>>> 2.17.1 >>>> >>> >> >
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c index 9ecfe13157c0..6658e927b1f5 100644 --- a/drivers/pci/pci-sysfs.c +++ b/drivers/pci/pci-sysfs.c @@ -218,6 +218,18 @@ static ssize_t current_link_width_show(struct device *dev, } static DEVICE_ATTR_RO(current_link_width); +static ssize_t available_bandwidth_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct pci_dev *pci_dev = to_pci_dev(dev); + u32 bw_avail; + + bw_avail = pcie_bandwidth_available(pci_dev, NULL, NULL, NULL); + + return sprintf(buf, "%u.%03u Gb/s\n", bw_avail / 1000, bw_avail % 1000); +} +static DEVICE_ATTR_RO(available_bandwidth); + static ssize_t secondary_bus_number_show(struct device *dev, struct device_attribute *attr, char *buf) @@ -786,6 +798,7 @@ static struct attribute *pcie_dev_attrs[] = { &dev_attr_current_link_width.attr, &dev_attr_max_link_width.attr, &dev_attr_max_link_speed.attr, + &dev_attr_available_bandwidth.attr, NULL, };
For certain bandwidth-critical devices (e.g. multi-port network cards) it is useful to know the available bandwidth to the root complex. This information is only available via the system log, which doesn't account for link degradation after probing. With a sysfs attribute, we can computes the bandwidth on-demand, and will detect degraded links. Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com> --- drivers/pci/pci-sysfs.c | 13 +++++++++++++ 1 file changed, 13 insertions(+)