Messages in this thread | | | Subject | Re: [RFC][PATCH 00/10] taskstats: Enhancements for precise accounting | From | Michael Holzheu <> | Date | Fri, 24 Sep 2010 11:10:15 +0200 |
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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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