Hi Maciej,
On 6/26/24 12:09 AM, Maciej Wieczor-Retman wrote:
Hello!,
On 2024-06-25 at 09:28:55 -0700, Reinette Chatre wrote:
Hi Maciej,
On 6/25/24 4:04 AM, Maciej Wieczor-Retman wrote:
Hello,
sorry it took me so long to get back to this. I prepared the next version with
your comments applied and Tony's replies taken into account.
Thank you very much for sticking with this.
I wanted to briefly discuss this before posting:
On 2024-05-30 at 16:07:29 -0700, Reinette Chatre wrote:
On 5/15/24 4:18 AM, Maciej Wieczor-Retman wrote:
+ return 1;
+ }
+
+ for (i = 1; i <= MAX_SNC ; i++) {
+ if (i * node_cpus >= cache_cpus)
+ return i;
+ }
This is not obvious to me. From the function comments this seems to address the
scenarios when CPUs from other nodes are offline. It is not clear to me how
this loop addresses this. For example, let's say there are four SNC nodes
associated with a cache and only the node0 CPUs are online. The above would
detect this as "1", not "4", if I read this right?
I wonder if it may not be easier to just follow what the kernel does
(in the new version).
User space can learn the number of online and present CPUs from
/sys/devices/system/cpu/online and /sys/devices/system/cpu/present
respectively. A simple string compare of the contents can be used to
determine if they are identical and a warning can be printed if they are not.
With a warning when accurate detection cannot be done the simple
check will do.
Could you please add an informational message indicating how many SNC nodes
were indeed detected?
Should the information "how many SNC nodes are detected?" get printed every time
(by which I mean at the end of CMT and MBM tests) or only when we get the error
"SNC enabled but kernel doesn't support it" happens? Of course in the first case
if there is only 1 node detected nothing would be printed to avoid noise.
I agree that it is not needed to print something about SNC if it is disabled.
hmmm ... so SNC impacts every test but it is only detected by default during CAT
and CMT test, with MBA and MBM "detection" only triggered if the test fails?
Yes, snc_ways() ran before starting CAT and CMT to adjust cache size variable.
And then after CAT,CMT,MBM and MBA if the return value indicated failure.
What if the "SNC detection" is moved to be within run_single_test() but instead of
repeating the detection from scratch every time it rather works like get_vendor()
where the full detection is only done on first attempt? run_single_test() can detect if
SNC is enabled and (if number of SNC nodes > 1) print an informational message
that is inherited by all tests.
Any test that needs to know the number of SNC nodes can continue to use the
same function used for detection (that only does actual detection once).
What do you think?
I think running the detection once at the start and then reusing the results is
a good idea. You're proposing adding a value (global or passed through all the
tests) that would get initialized on the first run_single_test()?
I was thinking about a solution similar to get_vendor() that uses a static local
variable. A global variable could work also.
And then the SNC status (if enabled) + a warning if the detection could be wrong
(because of the online/present cpus ratio) would happen before the test runs?
I do not think this was part of previous tests, but yes, this is a concern where
a warning would be helpful.
On the warning placement I think it should be moved out of being printed only on
failure. I did some experiments using "chcpu" to enable/disable cores and then
run selftests. They didn't have any problems succeeding even though SNC
detection detected different mode every time (I added a printf() around the line
I am not surprised here since there has not been much tuning of the CAT test.
that cache size is modified to show what SNC mode is detected). While I
understand these tests shouldn't fail since they just use a different portion of
the cache I think the user should be informed it's not really NUMA aware if the
detection was wrong:
Seems like there are two warnings to consider:
(a) SNC detection may be wrong.
(b) If SNC is enabled and kernel does not support SNC then the tests may fail.
For (a) I think that it is possible to know when SNC detection may be wrong. A test
similar to the kernel test that compares the "online" and "present" CPUs [1] can
be used. The /sys/devices/system/cpu/online and /sys/devices/system/cpu/present
files are available for this. A simpler way may be to just print a warning if
/sys/devices/system/cpu/offline is not empty and set the number of SNC nodes
to 1. Instead, a new "snc_unreliable" global can be set that can be used
to print additional information during test failure.
I do think that it is fair to print all the SNC details during detection but
I am concerned that those messages will be out of sight during test failures
and I thus do think it is useful to have extra information during test
failure.
Reinette
[1] https://lore.kernel.org/lkml/20240621223859.43471-18-tony.luck@xxxxxxxxx/
