On 10/18/2024 9:59 PM, Michael Kelley wrote: > From: Easwar Hariharan <eahariha@xxxxxxxxxxxxxxxxxxx> Sent: Friday, October 18, 2024 3:50 PM >> >> On 10/18/2024 12:54 AM, Praveen Kumar wrote: >>> On 17-10-2024 04:07, Easwar Hariharan wrote: >>>> We have several places where timeouts are open-coded as N (seconds) * HZ, >>>> but best practice is to use msecs_to_jiffies(). Convert the timeouts to >>>> make them HZ invariant. >>>>> Signed-off-by: Easwar Hariharan <eahariha@xxxxxxxxxxxxxxxxxxx> >>>> --- >>>> drivers/hv/hv_balloon.c | 9 +++++---- >>>> drivers/hv/hv_kvp.c | 4 ++-- >>>> drivers/hv/hv_snapshot.c | 6 ++++-- >>>> drivers/hv/vmbus_drv.c | 2 +- >>>> 4 files changed, 12 insertions(+), 9 deletions(-) >>>> >>>> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c >>>> index c38dcdfcb914d..3017d41f12681 100644 >>>> --- a/drivers/hv/hv_balloon.c >>>> +++ b/drivers/hv/hv_balloon.c >>>> @@ -756,7 +756,7 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size, >>>> * adding succeeded, it is ok to proceed even if the memory was >>>> * not onlined in time. >>>> */ >>>> - wait_for_completion_timeout(&dm_device.ol_waitevent, 5 * HZ); >>>> + wait_for_completion_timeout(&dm_device.ol_waitevent, msecs_to_jiffies(5 * 1000)); >>> >>> Is it correct to convert HZ to 1000 ? >>> Also, how are you testing these changes ? >>> >> >> It's a conversion of milliseconds to seconds, rather than HZ to 1000. :) >> msecs_to_jiffies() handles the conversion to jiffies with HZ. As Naman >> mentioned, this could be equivalently written as 5 * MSECS_PER_SEC, and >> would probably be more readable. On testing, this is only >> compile-tested, and that's part of the reason why it's an RFC, since I'm >> not 100% sure every one of these timeouts is measured in seconds. Hoping >> for folks more familiar with the code to take a look. >> > > I believe the current code is correct. Two things: > > 1) The values multiplied by HZ are indeed in seconds. The number of > seconds are somewhat arbitrary in some of the cases, so you might > argue for a different number of seconds. But as coded, the values > are in seconds. Thanks for reviewing, Michael, and for the confirmation. > > 2) Unless I'm missing something, the current code uses the correct > timeout regardless of the value of HZ because the number of jiffies > per second *is* HZ. > > As such, it might help to be explicit in the commit message that this > patch isn't fixing any bugs. Will do. > As the commit message says, the patch is > to bring the code into conformance with best practices. I presume > the best practice you reference is described in > Documentation/scheduler/completion.rst. > > I don't understand the statement about making the code "HZ invariant", > which I think came from the aforementioned documentation. Per > my #2 above, I think the existing code is already "HZ invariant", at > least for how I would interpret "HZ invariant". > That's right, both the best practice and the statement of HZ-invariance came from the scheduler documentation you pointed out. While I can't find the source with a quick search right now, I understand that HZ varies with CPU frequency and I figure that's what the statement is referring to. Unfortunately, there wasn't any discussion on HZ-invariance I can find on the lore thread where the statement was added. [1] It seems to be one of those "it's so self explanatory it doesn't warrant a mention" unless you're one of today's 10,000. [2] [1] https://lore.kernel.org/all/1539183392-239389-1-git-send-email-john.garry@xxxxxxxxxx/T/#u [2] https://xkcd.com/1053/ > Regardless of the meaning of "HZ invariant", I agree with the idea of > eliminating the use of HZ in cases like this, and letting msecs_to_jiffies() > handle it. Unfortunately, converting from "5 * HZ" to > "msecs_to_jiffies(5 * 1000)" makes the code really clunky. I would > advocate for adding something like this to include/linux/jiffies.h: > > #define secs_to_jiffies(secs) msecs_to_jiffies((secs) * 1000) > > and then using secs_to_jiffies() for all the cases in this patch. That > reduces the clunkiness. But maybe somebody in the past tried to > add secs_to_jiffies() and got shot down -- I don't know. It seems like > an obvious thing to add .... > > Michael
