Re: [RFC] [PATCH 0/5] procfs: reduce duplication by using symlinks

Jeff Mahoney
2019-03-21T18:30:12Z

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On 4/24/18 10:14 AM, Eric W. Biederman wrote:
> jeffm@suse.com writes:
>
>> From: Jeff Mahoney <jeffm@suse.com>
>>
>> Hi all -
>>
>> I recently encountered a customer issue where, on a machine with many TiB
>> of memory and a few hundred cores, after a task with a few thousand threads
>> and hundreds of files open exited, the system would softlockup. That
>> issue was (is still) being addressed by Nik Borisov's patch to add a
>> cond_resched call to shrink_dentry_list. The underlying issue is still
>> there, though. We just don't complain as loudly. When a huge task
>> exits, now the system is more or less unresponsive for about eight
>> minutes. All CPUs are pinned and every one of them is going through
>> dentry and inode eviction for the procfs files associated with each
>> thread. It's made worse by every CPU contending on the super's
>> inode list lock.
>>
>> The numbers get big. My test case was 4096 threads with 16384 files
>> open. It's a contrived example, but not that far off from the actual
>> customer case. In this case, a simple "find /proc" would create around
>> 300 million dentry/inode pairs. More practically, lsof(1) does it too,
>> it just takes longer. On smaller systems, memory pressure starts pushing
>> them out. Memory pressure isn't really an issue on this machine, so we
>> end up using well over 100GB for proc files. It's the combination of
>> the wasted CPU cycles in teardown and the wasted memory at runtime that
>> pushed me to take this approach.
>>
>> The biggest culprit is the "fd" and "fdinfo" directories, but those are
>> made worse by there being multiple copies of them even for the same
>> task without threads getting involved:
>>
>> - /proc/pid/fd and /proc/pid/task/pid/fd are identical but share no
>> resources.
>>
>> - Every /proc/pid/task/*/fd directory in a thread group has identical
>> contents (unless unshare(CLONE_FILES) was called), but share no
>> resources.
>>
>> - If we do a lookup like /proc/pid/fd on a member of a thread group,
>> we'll get a valid directory. Inside, there will be a complete
>> copy of /proc/pid/task/* just like in /proc/tgid/task. Again,
>> nothing is shared.
>>
>> This patch set reduces some (most) of the duplication by conditionally
>> replacing some of the directories with symbolic links to copies that are
>> identical.
>>
>> 1) Eliminate the duplication of the task directories between threads.
>> The task directory belongs to the thread leader and the threads
>> link to it: e.g. /proc/915/task -> ../910/task This mainly
>> reduces duplication when individual threads are looked up directly
>> at the tgid level. The impact varies based on the number of threads.
>> The user has to go out of their way in order to mess up their system
>> in this way. But if they were so inclined, they could create ~550
>> billion inodes and dentries using the test case.
>>
>> 2) Eliminate the duplication of directories that are created identically
>> between the tgid-level pid directory and its task directory: fd,
>> fdinfo, ns, net, attr. There is obviously more duplication between
>> the two directories, but replacing a file with a symbolic link
>> doesn't get us anything. This reduces the number of files associated
>> with fd and fdinfo by half if threads aren't involved.
>>
>> 3) Eliminate the duplication of fd and fdinfo directories among threads
>> that share a files_struct. We check at directory creation time if
>> the task is a group leader and if not, whether it shares ->files with
>> the group leader. If so, we create a symbolic link to ../tgid/fd*.
>> We use a d_revalidate callback to check whether the thread has called
>> unshare(CLONE_FILES) and, if so, fail the revalidation for the symlink.
>> Upon re-lookup, a directory will be created in its place. This is
>> pretty simple, so if the thread group leader calls unshare, all threads
>> get directories.
>>
>> With these patches applied, running the same testcase, the proc_inode
>> cache only gets to about 600k objects, which is about 99.7% fewer. I
>> get that procfs isn't supposed to be scalable, but this is kind of
>> extreme. :)
>>
>> Finally, I'm not a procfs expert. I'm posting this as an RFC for folks
>> with more knowledge of the details to pick it apart. The biggest is that
>> I'm not sure if any tools depend on any of these things being directories
>> instead of symlinks. I'd hope not, but I don't have the answer. I'm
>> sure there are corner cases I'm missing. Hopefully, it's not just flat
>> out broken since this is a problem that does need solving.
>>
>> Now I'll go put on the fireproof suit.

Thanks for your comments. This ended up having to get back-burnered but
I've finally found some time to get back to it. I have new patches that
don't treat each entry as a special case and makes more sense, IMO.
They're not worth posting yet since some of the issues below remain.

> This needs to be tested against at least apparmor to see if this breaks
> common policies. Changing files to symlinks in proc has a bad habit of
> either breaking apparmor policies or userspace assumptions. Symbolic
> links are unfortunately visible to userspace.

AppArmor uses the @{pids} var in profiles that translates to a numeric
regex. That means that /proc/pid/task -> /proc/tgid/task won't break
profiles but /proc/pid/fdinfo -> /proc/pid/task/tgid/fdinfo will break.
Apparmor doesn't have a follow_link hook at all, so all that matters is
the final path. SELinux does have a follow_link hook, but I'm not
familiar enough with it to know whether introducing a symlink in proc
will make a difference.

I've dropped the /proc/pid/{dirs} -> /proc/pid/task/pid/{dirs} part
since that clearly won't work.

> Further the proc structure is tgid/task/tid where the leaf directories
> are per thread.

Yes, but threads are still in /proc for lookup at the tgid level even if
they don't show up in readdir.

> We more likely could get away with some magic symlinks (that would not
> be user visible) rather than actual symlinks.

I think I'm missing something here. Aren't magic symlinks still
represented to the user as symlinks?

> So I think you are probably on the right track to reduce the memory
> usage but I think some more work will be needed to make it transparently
> backwards compatible.

Yeah, that's going to be the big hiccup. I think I've resolved the
biggest issue with AppArmor, but I don't think the problem is solvable
without introducing symlinks.

-Jeff

--
Jeff Mahoney
SUSE Labs

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