Re: [RFC][PATCH 00/10] taskstats: Enhancements for precise accounting

[Balbir Singh]

* Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx> [2010-09-24 11:10:15]:

> Hello Andrew,
> 
> On Thu, 2010-09-23 at 13:11 -0700, Andrew Morton wrote:
> > > GOALS OF THIS PATCH SET
> > > -----------------------
> > > The intention of this patch set is to provide better support for tools like
> > > top. The goal is to:
> > > 
> > > * provide a task snapshot mechanism where we can get a consistent view of
> > >   all running tasks.
> > > * provide a transport mechanism that does not require a lot of system calls
> > >   and that allows implementing low CPU overhead task monitoring.
> > > * provide microsecond CPU time granularity.
> > 
> > This is a big change!  If this is done right then we're heading in the
> > direction of deprecating the longstanding way in which userspace
> > observes the state of Linux processes and we're recommending that the
> > whole world migrate to taskstats.  I think?
>

Wouldn't I love that :)
 
> Or it can be used as alternative. Since procfs has its drawbacks (e.g.
> performance) an alternative could be helpful. 
> 
> And the taskstats interface with the TASKSTATS_CMD_ATTR_PID command
> already exists and can be used. So we already have a second mechanism to
> query tasks accounting information besides of procfs.
> 

Yes, an alternative for simple data extraction without having to write
network code to extract it.

> > 
> > If so, much chin-scratching will be needed, coordination with
> > util-linux people, etc.
> 
> I agree.
> 
> > We'd need to think about the implications of taskstats versioning.  It
> > _is_ a versioned interface, so people can't just go and toss random new
> > stuff in there at will - it's not like adding a new procfs file, or
> > adding a new line to an existing one.  I don't know if that's likely to
> > be a significant problem.
> 
> I already thought about that problem. Another problem is that depending
> on the kernel config options, some taskstats fields may be not
> initialized. E.g. CONFIG_TASK_DELAY_ACCT or CONFIG_TASK_XACCT. Currently
> there does not exist a good interface to userspace to query which fields
> are valid.
> 
> Regarding the taskstats versions  I described a possible solution in the
> userspace tarball in the README.libtaskstats file:
> 
> The "struct taskstats" structure contains accounting information for one
> Linux task. This structure is defined in "/usr/include/linux/taskstats.h".
> With new kernel versions new fields can be added to that structure.
> In that case the kernel taskstats version number defined with the macro
> TASKSTATS_VERSION will be increased.
>
> The taskstats library distinguishes between two taskstats versions:
> * Kernel taskstats version (KV)
> * Program compile taskstats version (CV)
> 
> Depending on the taskstats version CV that is used for compiling the program,
> this version numbers can be different:
> * KV > CV:
>   The libtaskstats library only copies the CV taskstats fields and the fields
>   that belong to version > CV will be ignored.
> * KV < CV:
>   The libtaskstats library only copies the version KV fields and the fields
>   that belong to version > KV remain uninitialized.
> 
> If a program wants to support multiple taskstats versions, this can be done
> using the ts_version() function and process fields according to that version
> number.
> 
> Example:
> 
>   if (ts_version() < 7) {
>          fprintf(stderr, "Error: kernel taskstats version too low\n");
>          exit(1);
>   }
>   if (ts_version() >= 7)
>          print_attrs_v7();
>   if (ts_version() >= 8)
>          print_attrs_v8();
> 
> In this example the program has to be compiled with a taskstats.h header file
> that has at least version 8.

Fair enough

> 
> > I worry that there's a dependency on CONFIG_NET?  If so then that's a
> > big problem because in N years time, 99% of the world will be using
> > taskstats, but a few embedded losers will be stuck using (and having to
> > support) the old tools.
> 
> Sure, but if we could add the /proc/taskstats approach, this dependency
> would not be there.
> 
> > 
> > > FIRST RESULTS
> > > -------------
> > > Together with this kernel patch set also user space code for a new top
> > > utility (ptop) is provided that exploits the new kernel infrastructure. See
> > > patch 10 for more details.
> > > 
> > > TEST1: System with many sleeping tasks
> > > 
> > >   for ((i=0; i < 1000; i++))
> > >   do
> > >          sleep 1000000 &
> > >   done
> > > 
> > >   # ptop_new_proc
> > > 
> > >              VVVV
> > >   pid   user  sys  ste  total  Name
> > >   (#)    (%)  (%)  (%)    (%)  (str)
> > >   541   0.37 2.39 0.10   2.87  top
> > >   3743  0.03 0.05 0.00   0.07  ptop_new_proc
> > >              ^^^^
> > > 
> > > Compared to the old top command that has to scan more than 1000 proc
> > > directories the new ptop consumes much less CPU time (0.05% system time
> > > on my s390 system).
> > 
> > How many CPUs does that system have?
> 
> The system is a virtual machine and has three CPUs.
> 
> > What's the `top' update period?  One second?
> 
> The update period is two seconds.
> 
> > So we're saying that a `top -d 1' consumes 2.4% of this
> > mystery-number-of-CPUs machine?  That's quite a lot.
> 
> When I run that testcase on my laptop, 2 CPUs (Intel Core 2 - 2.33GHz),
> I get about 1-2% system time for top.
> 
> > > PATCHSET OVERVIEW
> > > -----------------
> > > The code is not final and still has a few TODOs. But it is good enough for a
> > > first round of review. The following kernel patches are provided:
> > > 
> > > [01] Prepare-0: Use real microsecond granularity for taskstats CPU times.
> > > [02] Prepare-1: Restructure taskstats.c in order to be able to add new commands
> > >      more easily.
> > > [03] Prepare-2: Separate the finding of a task_struct by PID or TGID from
> > >      filling the taskstats.
> > > [04] Add new command "TASKSTATS_CMD_ATTR_PIDS" to get a snapshot of multiple
> > >      tasks.
> > > [05] Add procfs interface for taskstats commands. This allows to get a complete
> > >      and consistent snapshot with all tasks using two system calls (ioctl and
> > >      read). Transferring a snapshot of all running tasks is not possible using
> > >      the existing netlink interface, because there we have the socket buffer
> > >      size as restricting factor.
> > 
> > So this is a binary interface which uses an ioctl.  People don't like
> > ioctls.  Could we have triggered it with a write() instead?
> 
> The current idea is the following:
> 
> 1. Open /proc/taskstats
> 2. Set the requested command (e.g. TASKSTATS_CMD_ATTR_PIDS) using
>    an ioctl. For the TASKSTATS_CMD_ATTR_PIDS ioctl the following
>    structure is sent:
> 
>    struct taskstats_cmd_pids {
>         __u64   time_ns;
>         __u32   pid;
>         __u32   cnt;
>    };
> 
> 3. After the command is defined, with a read() the command is executed
>    and the result is returned to the user's read buffer.
> 
> We could replace step 2 with a write, that transfers the command.
>

I don't like ioctls either, write sounds interesting.
 
> > Does this have the potential to save us from the CONFIG_NET=n problem?
> 
> Yes
> 
> > > [06] Add TGID to taskstats.
> > > [07] Add steal time per task accounting.
> > > [08] Add cumulative CPU time (user, system and steal) to taskstats.
> > 
> > These didn't update the taskstats version number.  Should they have?
> 
> Patch 04/10 updates the taskstats version number from 7 to 8.
> I didn't want to update the version number with each patch.
> 
> > > [09] Fix exit CPU time accounting.
> > > 
> > > [10] Besides of the kernel patches also user space code is provided that
> > >      exploits the new kernel infrastructure. The user space code provides the
> > >      following:
> > >      1. A proposal for a taskstats user space library:
> > >         1.1 Based on netlink (requires libnl-devel-1.1-5)
> > >         2.1 Based on the new /proc/taskstats interface (see [05])
> > >      2. A proposal for a task snapshot library based on taskstats library (1.1)
> > 
> > ooh, excellent.  A standardised userspace access library.
> 
> Yes, at least a proposal for that.
> 
> > >      3. A new tool "ptop" (precise top) that uses the libraries
> > 
> > Talk to me about namespaces, please.  A lot of the new code involves
> > PIDs, but PIDs are not system-wide unique.  A PID is relative to a PID
> > namespace.  Does everything Just Work?  When userspace sends a PID to
> > the kernel, that PID is assumed to be within the sending process's PID
> > namespace?  If so, then please spell it all out in the changelogs.  If
> > not then that is a problem!
> 
> To be honest, I have not tested that. I assumed that the current
> taskstats code does this correctly. E.g. it uses find_task_by_vpid() for
> TASKSTATS_CMD_ATTR_PID and this function uses
> "current->nsproxy->pid_ns". So I would assume that we get only tasks
> from the caller's namespace. The new TASKSTATS_CMD_ATTR_PIDS command
> also uses also only functions with "current->nsproxy->pid_ns".
> 
> > If I can only observe processes in my PID namespace then is that a
> > problem?  Should I be allowed to observe another PID namespace's
> > processes?  I assume so, because I might be root.  If so, how is that
> > to be done?
> 
> Good question. Probably I have to learn a bit more about the PID
> namespace implementation. Are PIDs over all namespaces unique?
> 
>

I think the namespaces are OK, we might peep into namespaces nested
within the current one, but that is legal today. 

-- 
	Three Cheers,
	Balbir
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