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

[Jeff Mahoney]

On 4/24/18 10:14 AM, Eric W. Biederman wrote:
> jeffm@xxxxxxxx writes:
> 
>> From: Jeff Mahoney <jeffm@xxxxxxxx>
>>
>> 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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