Re: [PATCH v7 00/11] extend task comm from 16 to 24

[Michał Mirosław]

On Sat, Nov 06, 2021 at 05:12:24PM +0800, Yafang Shao wrote:
> On Sat, Nov 6, 2021 at 7:57 AM Michał Mirosław <mirq-linux@xxxxxxxxxxxx> wrote:
> >
> > On Fri, Nov 05, 2021 at 02:34:58PM +0800, Yafang Shao wrote:
> > > On Thu, Nov 4, 2021 at 9:37 AM Michał Mirosław <mirq-linux@xxxxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Nov 01, 2021 at 06:04:08AM +0000, Yafang Shao wrote:
> > > > > There're many truncated kthreads in the kernel, which may make trouble
> > > > > for the user, for example, the user can't get detailed device
> > > > > information from the task comm.
> > > > >
> > > > > This patchset tries to improve this problem fundamentally by extending
> > > > > the task comm size from 16 to 24, which is a very simple way.
> > > > [...]
> > > >
> > > > Hi,
> > > >
> > > > I've tried something like this a few years back. My attempt got mostly
> > > > lost in the mailing lists, but I'm still carrying the patches in my
> > > > tree [1]. My target was userspace thread names, and it turned out more
> > > > involved than I had time for.
> > > >
> > > > [1] https://rere.qmqm.pl/git/?p=linux;a=commit;h=2c3814268caf2b1fee6d1a0b61fd1730ce135d4a
> > > >     and its parents
> > > >
> > >
> > > Hi Michal,
> > >
> > > Thanks for the information.
> > >
> > > I have looked through your patches.  It seems to contain six patches
> > > now and can be divided into three parts per my understanding.
> > >
> > > 1. extend task comm len
> > > This parts contains below 4 patches:
> > > [prctl: prepare for bigger
> > > TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=cfd99db9cf911bb4d106889aeba1dfe89b6527d0)
> > > [bluetooth: prepare for bigger
> > > TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=ba2805f5196865b81cc6fc938ea53af2c7c2c892)
> > > [taskstats: prepare for bigger
> > > TASK_COMM_LEN](https://rere.qmqm.pl/git/?p=linux;a=commit;h=4d29bfedc57b36607915a0171f4864ec504908ca)
> > > [mm: make TASK_COMM_LEN
> > > configurable](https://rere.qmqm.pl/git/?p=linux;a=commit;h=362acc35582445174589184c738c4d86ec7d174b)
> > >
> > > What kind of userspace issues makes you extend the task comm length ?
> > > Why not just use /proc/[pid]/cmdline ?
> >
> > This was to enable longer thread names (as set by pthread_setname_np()).
> > Currently its 16 bytes, and that's too short for e.g. Chrome's or Firefox'es
> > threads. I believe that FreeBSD has 32-byte limit and so I expect that
> > major portable code is already prepared for bigger thread names.
> >
> 
> The comm len in FreeBSD is (19 + 1) bytes[1], but that is still larger
> than Linux :)
> The task comm is short for many applications, that is why cmdline is
> introduced per my understanding, but pthread_{set, get}name_np() is
> reading/writing the comm or via prctl(2) rather than reading/writing
> the cmdline...
> 
> Is the truncated Chrome or Firefox thread comm really harmful or is
> extending the task comm just for portable?
> Could you pls show me some examples if the short comm is really harmful?
> 
> Per my understanding, if the short comm is harmful to applications
> then it is worth extending it.
> But if it is only for portable code, it may not be worth extending it.
> 
> [1]. https://github.com/freebsd/freebsd-src/blob/main/sys/sys/param.h#L126

I don't think it is harmful as in exposing a bug or something. It's just
inconvenient when debugging a system where you can't differentiate
between threads because their names have been cut too short.

Best Regards
Michał Mirosław