From: Nathan Lynch <email@example.com> To: Gautham R Shenoy <firstname.lastname@example.org> Cc: Tyrel Datwyler <email@example.com>, firstname.lastname@example.org, Kamalesh Babulal <email@example.com>, Vaidyanathan Srinivasan <firstname.lastname@example.org>, email@example.com, "Naveen N. Rao" <firstname.lastname@example.org> Subject: Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr Date: Fri, 03 Apr 2020 13:10:02 -0500 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <20200403062818.GB9066@in.ibm.com> Gautham R Shenoy <firstname.lastname@example.org> writes: > On Thu, Apr 02, 2020 at 01:04:34PM +0530, Naveen N. Rao wrote: >> >> >> >>I wonder if we should introduce a sysctl interface to control thresholding. >> >>It can default to 0, which disables thresholding so that the existing >> >>behavior continues. Applications (lparstat) can optionally set it to suit >> >>their use. >> > >> >We would be introducing 3 new sysfs interfaces that way instead of >> >two. >> > >> >/sys/devices/system/cpu/purr_spurr_staleness >> >/sys/devices/system/cpu/cpuX/idle_purr >> >/sys/devices/system/cpu/cpuX/idle_spurr >> > >> >I don't have a problem with this. Nathan, Michael, thoughts on this? No, I don't think this warrants a tunable when the issue it's intended to address is still a bit speculative at this point. (Also, note that this would be a system-wide value, but you could have multiple concurrent users of the interface with different needs.) >> >The alternative is to have a procfs interface, something like >> >/proc/powerpc/resource_util_stats >> > >> >which gives a listing similar to /proc/stat, i.e >> > >> > CPUX <purr> <idle_purr> <spurr> <idle_spurr> >> > >> >Even in this case, the values can be obtained in one-shot with a >> >single IPI and be printed in the row corresponding to the CPU. >> >> Right -- and that would be optimal requiring a single system call, at the >> cost of using a legacy interface. >> >> The other option would be to drop this patch and to just go with patches 1-5 >> introducing the new sysfs interfaces for idle_[s]purr. It isn't entirely >> clear how often this would be used, or its actual impact. We can perhaps >> consider this optimization if and when this causes problems... > > I am ok with that. We can revisit the problem if IPI noise becomes > noticable. However, if Nathan or Michael feel that this problem is > better solved now, than leaving it for the future, we will have to > take a call on what the interface is going to be. While I maintain some concern about the overhead on larger LPARs (150us per CPU works out to ~0.15s total to serially sample 1024 CPUs, ~0.3s for 2048 and so on), I am OK with the straightforward addition of the attributes without any batching or sampling thresholds behind the scenes for now. I appreciate your consideration of the issue. If this turns out to be too inefficient then I think we should consider a non-sysfs mechanism such as chardev+ioctl.
prev parent reply other threads:[~2020-04-03 18:10 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-27 11:32 [PATCH v4 0/6] [PATCH v4 0/6] Track and expose idle PURR and SPURR ticks Gautham R. Shenoy 2020-03-27 11:32 ` [PATCH v4 1/6] powerpc: Move idle_loop_prolog()/epilog() functions to header file Gautham R. Shenoy 2020-03-27 11:32 ` [PATCH v4 2/6] powerpc/idle: Add accessor function to always read latest idle PURR Gautham R. Shenoy 2020-04-01 9:42 ` Naveen N. Rao 2020-04-03 6:15 ` Gautham R Shenoy 2020-04-03 10:34 ` Naveen N. Rao 2020-04-03 11:24 ` Gautham R Shenoy 2020-03-27 11:32 ` [PATCH v4 3/6] powerpc/pseries: Account for SPURR ticks on idle CPUs Gautham R. Shenoy 2020-03-27 11:32 ` [PATCH v4 4/6] powerpc/sysfs: Show idle_purr and idle_spurr for every CPU Gautham R. Shenoy 2020-03-27 11:32 ` [PATCH v4 5/6] Documentation: Document sysfs interfaces purr, spurr, idle_purr, idle_spurr Gautham R. Shenoy 2020-04-01 9:45 ` Naveen N. Rao 2020-03-27 11:32 ` [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr Gautham R. Shenoy 2020-04-01 9:58 ` Naveen N. Rao 2020-04-01 12:01 ` Gautham R Shenoy 2020-04-02 7:34 ` Naveen N. Rao 2020-04-03 6:28 ` Gautham R Shenoy 2020-04-03 18:10 ` Nathan Lynch [this message]
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --subject='Re: [PATCH v4 6/6] pseries/sysfs: Minimise IPI noise while reading [idle_][s]purr' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).