Re: [PATCH] vfs: shave work on failed file open

[Mateusz Guzik]

On 9/26/23, John Stoffel <john@xxxxxxxxxxx> wrote:
>>>>>> "Mateusz" == Mateusz Guzik <mjguzik@xxxxxxxxx> writes:
>
>> Failed opens (mostly ENOENT) legitimately happen a lot, for example here
>> are stats from stracing kernel build for few seconds (strace -fc make):
>
>>   % time     seconds  usecs/call     calls    errors syscall
>>   ------ ----------- ----------- --------- --------- ------------------
>>     0.76    0.076233           5     15040      3688 openat
>
>> (this is tons of header files tried in different paths)
>
>> Apart from a rare corner case where the file object is fully constructed
>> and we need to abort, there is a lot of overhead which can be avoided.
>
>> Most notably delegation of freeing to task_work, which comes with an
>> enormous cost (see 021a160abf62 ("fs: use __fput_sync in close(2)" for
>> an example).
>
>> Benched with will-it-scale with a custom testcase based on
>> tests/open1.c:
>> [snip]
>>         while (1) {
>>                 int fd = open("/tmp/nonexistent", O_RDONLY);
>>                 assert(fd == -1);
>
>>                 (*iterations)++;
>>         }
>> [/snip]
>
>> Sapphire Rapids, one worker in single-threaded case (ops/s):
>> before:	1950013
>> after:	2914973 (+49%)
>
>
> So what are the times in a multi-threaded case?  Just wondering what
> happens if you have a bunch of makes or other jobs like that all
> running at once.
>

On my kernel they heavily bottleneck on apparmor, I already mailed the author:
https://lore.kernel.org/all/CAGudoHFfG7mARwSqcoLNwV81-KX4Bici5FQHjoNG4f9m83oLyg@xxxxxxxxxxxxxx/

maybe i'll hack up the fix

When running without that LSM and on *stock* kernel it heavily
bottlenecks somewhere in bowels of SLUB + RCU.

Without LSM and with the patch it scales almost perfectly, as one would expect.

I don't have numbers nor perf output handy.

>
>> Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
>> ---
>>  fs/file_table.c      | 39 +++++++++++++++++++++++++++++++++++++++
>>  fs/namei.c           |  2 +-
>>  include/linux/file.h |  1 +
>>  3 files changed, 41 insertions(+), 1 deletion(-)
>
>> diff --git a/fs/file_table.c b/fs/file_table.c
>> index ee21b3da9d08..320dc1f9aa0e 100644
>> --- a/fs/file_table.c
>> +++ b/fs/file_table.c
>> @@ -82,6 +82,16 @@ static inline void file_free(struct file *f)
>>  	call_rcu(&f->f_rcuhead, file_free_rcu);
>>  }
>
>> +static inline void file_free_badopen(struct file *f)
>> +{
>> +	BUG_ON(f->f_mode & (FMODE_BACKING | FMODE_OPENED));
>
> eww... what a BUG_ON() here?  This seems *way* overkill to crash the
> system here, and you don't even check if f exists first as well, since
> I assume the caller checks it or already knows it?
>
> Why not just return an error here and keep going?  What happens if you do?
>

The only caller already checked these flags, so I think BUGing out is prudent.

>
>> +	security_file_free(f);
>> +	put_cred(f->f_cred);
>> +	if (likely(!(f->f_mode & FMODE_NOACCOUNT)))
>> +		percpu_counter_dec(&nr_files);
>> +	kmem_cache_free(filp_cachep, f);
>> +}
>> +
>>  /*
>>   * Return the total number of open files in the system
>>   */
>> @@ -468,6 +478,35 @@ void __fput_sync(struct file *file)
>>  EXPORT_SYMBOL(fput);
>>  EXPORT_SYMBOL(__fput_sync);
>
>> +/*
>> + * Clean up after failing to open (e.g., open(2) returns with -ENOENT).
>> + *
>> + * This represents opportunities to shave on work in the common case
>> compared
>> + * to the usual fput:
>> + * 1. vast majority of the time FMODE_OPENED is not set, meaning there is
>> no
>> + *    need to delegate to task_work
>> + * 2. if the above holds then we are guaranteed we have the only
>> reference with
>> + *    nobody else seeing the file, thus no need to use atomics to release
>> it
>> + * 3. then there is no need to delegate freeing to RCU
>> + */
>> +void fput_badopen(struct file *file)
>> +{
>> +	if (unlikely(file->f_mode & (FMODE_BACKING | FMODE_OPENED))) {
>> +		fput(file);
>> +		return;
>> +	}
>> +
>> +	if (WARN_ON(atomic_long_read(&file->f_count) != 1)) {
>> +		fput(file);
>> +		return;
>> +	}
>> +
>> +	/* zero out the ref count to appease possible asserts */
>> +	atomic_long_set(&file->f_count, 0);
>> +	file_free_badopen(file);
>> +}
>> +EXPORT_SYMBOL(fput_badopen);
>> +
>>  void __init files_init(void)
>>  {
>>  	filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 567ee547492b..67579fe30b28 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -3802,7 +3802,7 @@ static struct file *path_openat(struct nameidata
>> *nd,
>>  		WARN_ON(1);
>>  		error = -EINVAL;
>>  	}
>> -	fput(file);
>> +	fput_badopen(file);
>>  	if (error == -EOPENSTALE) {
>>  		if (flags & LOOKUP_RCU)
>>  			error = -ECHILD;
>> diff --git a/include/linux/file.h b/include/linux/file.h
>> index 6e9099d29343..96300e27d9a8 100644
>> --- a/include/linux/file.h
>> +++ b/include/linux/file.h
>> @@ -15,6 +15,7 @@
>>  struct file;
>
>>  extern void fput(struct file *);
>> +extern void fput_badopen(struct file *);
>
>>  struct file_operations;
>>  struct task_struct;
>> --
>> 2.39.2
>
>


-- 
Mateusz Guzik <mjguzik gmail.com>