On Tue, Aug 18, 2020 at 11:45 PM peter enderborg <peter.enderborg@sony.com> wrote:
On 8/18/20 7:53 PM, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:06 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 11:01:35PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 10:44 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 10:24:13PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 8:06 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 08:00:11PM +0530, Muni Sekhar wrote:
Hi all,
I’ve two identical Linux systems with only kernel differences. What are the differences in the kernels? You didn't answer this question, is this the same kernel source being compared here? Same version? Same compiler? Everything identical? Both systems are having exactly the same hardware configuration. Compiler and kernel versions are different. One system has Ubuntu 16.04.4 LTS(4.4.0-66-generic kernel with gcc version 5.4.0) kernel and the other one has Ubuntu 18.04.4 LTS(4.15.0-91-generic kernel with gcc version 7.5.0). Those are _very_ different kernel versions, with many years and tens of thousands of different changes between them.
Hopefully the newer kernel is faster, so just stick with that :) But unfortunately the newer kernel is very slow, that is the reason for starting this investigation :) Any type of help, and guidelines to dive deeper will be highly appreciated.
On the 4.4 kernel you dont have
+CONFIG_RETPOLINE=y +CONFIG_INTEL_RDT=y
Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above two config options. What analysis can be done to narrow down the root cause? Any example of reference could be helpful to understand.
And your base is very different two.
Try to use mainline on both system and see.
You can also use the same base kernel version from ubuntu and
run your test.
greg k-h
-- Thanks, Sekhar
On 19/08/2020 10:16, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:45 PM peter enderborg <peter.enderborg@sony.com> wrote: [...]
On the 4.4 kernel you dont have
+CONFIG_RETPOLINE=y +CONFIG_INTEL_RDT=y Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above two config options. What analysis can be done to narrow down the root cause? Any example of reference could be helpful to understand.
I haven't checked the date of the older kernel but Spectre+ Meltdown mitigation costs a lot (20%-30% speed IIRC, out of the top of my head). MfG, Bernd -- There is no cloud, just other people computers. -- https://static.fsf.org/nosvn/stickers/thereisnocloud.svg
From: Bernd Petrovitsch
Sent: 19 August 2020 11:22
On 19/08/2020 10:16, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:45 PM peter enderborg <peter.enderborg@sony.com> wrote: [...]
On the 4.4 kernel you dont have
+CONFIG_RETPOLINE=y +CONFIG_INTEL_RDT=y Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above two config options. What analysis can be done to narrow down the root cause? Any example of reference could be helpful to understand.
I haven't checked the date of the older kernel but Spectre+ Meltdown mitigation costs a lot (20%-30% speed IIRC, out of the top of my head).
It depends greatly on the benchmark. Page table separation has a massive effect on anything that does a lot of short system calls. There may be some mitigation in very recent hardware. And I can't remember if it gets disabled for 'root' processes (which can read kernel memory anyway). It is definitely worth building a kernel with the mitigations disabled - if only to check that they are responsible. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Wed, Aug 19, 2020 at 03:46:06PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:45 PM peter enderborg <peter.enderborg@sony.com> wrote:
On 8/18/20 7:53 PM, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:06 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 11:01:35PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 10:44 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 10:24:13PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 8:06 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 08:00:11PM +0530, Muni Sekhar wrote:
Hi all,
I’ve two identical Linux systems with only kernel differences. What are the differences in the kernels? You didn't answer this question, is this the same kernel source being compared here? Same version? Same compiler? Everything identical? Both systems are having exactly the same hardware configuration. Compiler and kernel versions are different. One system has Ubuntu 16.04.4 LTS(4.4.0-66-generic kernel with gcc version 5.4.0) kernel and the other one has Ubuntu 18.04.4 LTS(4.15.0-91-generic kernel with gcc version 7.5.0). Those are _very_ different kernel versions, with many years and tens of thousands of different changes between them.
Hopefully the newer kernel is faster, so just stick with that :) But unfortunately the newer kernel is very slow, that is the reason for starting this investigation :) Any type of help, and guidelines to dive deeper will be highly appreciated.
On the 4.4 kernel you dont have
+CONFIG_RETPOLINE=y +CONFIG_INTEL_RDT=y
Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above two config options. What analysis can be done to narrow down the root cause?
"root cause" is you are comparing a kernel without any spectre/meltdown mitigations, with one that has it. Those mitigations are known to have a huge slowdown on some benchmarks/workloads, as you have shown here. If you don't like this, you can disable these changes on newer kernels, if you really really know what you are doing, but I strongly do not recommend it. Also you can complain to your hardware vendors for selling you broken hardware :)
Any example of reference could be helpful to understand.
Look up Spectre and Meltdown for many many examples of what happened and what went wrong with chip designs and how we had to fix these things in the kernel a few years ago. greg k-h
On Wed, 19 Aug 2020 12:42:54 +0200, Greg KH said:
Look up Spectre and Meltdown for many many examples of what happened and what went wrong with chip designs and how we had to fix these things in the kernel a few years ago.
And I'm sure that nobody sane thinks we're done with security holes caused by speculative execution... :)
On Wed, Aug 19, 2020 at 12:43:52PM -0400, Valdis Klētnieks wrote:
On Wed, 19 Aug 2020 12:42:54 +0200, Greg KH said:
Look up Spectre and Meltdown for many many examples of what happened and what went wrong with chip designs and how we had to fix these things in the kernel a few years ago.
And I'm sure that nobody sane thinks we're done with security holes caused by speculative execution... :)
Hah, not at all, the patches keep flowing in, as do the microcode updates...
Em Wed, Aug 19, 2020 at 03:46:06PM +0530, Muni Sekhar escreveu:
On Tue, Aug 18, 2020 at 11:45 PM peter enderborg <peter.enderborg@sony.com> wrote:
On 8/18/20 7:53 PM, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 11:06 PM Greg KH <greg@kroah.com> wrote:
On Tue, Aug 18, 2020 at 11:01:35PM +0530, Muni Sekhar wrote:
On Tue, Aug 18, 2020 at 10:44 PM Greg KH <greg@kroah.com> wrote: Those are _very_ different kernel versions, with many years and tens of thousands of different changes between them.
Hopefully the newer kernel is faster, so just stick with that :) But unfortunately the newer kernel is very slow, that is the reason for starting this investigation :) Any type of help, and guidelines to dive deeper will be highly appreciated.
On the 4.4 kernel you dont have
+CONFIG_RETPOLINE=y +CONFIG_INTEL_RDT=y Thanks! That is helpful. Yes, I see 4.4 kernel don't have the above two config options. What analysis can be done to narrow down the root cause? Any example of reference could be helpful to understand.
This made me remember Lisbon: http://vger.kernel.org/~acme/perf/bpf-is-eating-the-world-dont-you-see-lpc-l... Greg was there when this slide was first shown... ;-) Look at the icache misses when the kernel is entered (syscall entry). Then add this to the kernel command line: "nospectre_v1 nospectre_v2" And see what happens: http://vger.kernel.org/~acme/perf/bpf-is-eating-the-world-dont-you-see-lpc-l... So using these (and any new ones related to disabling hardware flaw mitigation) maybe a way to analyse this, if you haven't figured this out yet, sorry for the huge delay in replying 8-) - Arnaldo
And your base is very different two.
Try to use mainline on both system and see.
You can also use the same base kernel version from ubuntu and
run your test.
greg k-h
-- Thanks, Sekhar
-- - Arnaldo
participants (6)
-
Arnaldo Carvalho de Melo -
Bernd Petrovitsch -
David Laight -
Greg KH -
Muni Sekhar -
Valdis Klētnieks