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?
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.
>
> 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.
> 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.
> 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?
Michael
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