Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
On Thu, Aug 27, 2020 at 03:28:48PM +0100, Al Viro wrote:
> On Thu, Aug 27, 2020 at 06:19:44PM +0800, Shaokun Zhang wrote:
> > From: Yuqi Jin
> >
> > It is well known that the performance of atomic_add is better than that of
> > atomic_cmpxchg.
> > The initial value of @f_count is 1. While @f_count is increased by 1 in
> > __fget_files, it will go through three phases: > 0, < 0, and = 0. When the
> > fixed value 0 is used as the condition for terminating the increase of 1,
> > only atomic_cmpxchg can be used. When we use < 0 as the condition for
> > stopping plus 1, we can use atomic_add to obtain better performance.
>
> Suppose another thread has just removed it from the descriptor table.
>
> > +static inline bool get_file_unless_negative(atomic_long_t *v, long a)
> > +{
> > + long c = atomic_long_read(v);
> > +
> > + if (c <= 0)
> > + return 0;
>
> Still 1. Now the other thread has gotten to dropping the last reference,
> decremented counter to zero and committed to freeing the struct file.
>
> > +
> > + return atomic_long_add_return(a, v) - 1;
>
> ... and you increment that sucker back to 1. Sure, you return 0, so the
> caller does nothing to that struct file. Which includes undoing the
> changes to its refecount.
>
> In the meanwhile, the third thread does fget on the same descriptor,
> and there we end up bumping the refcount to 2 and succeeding. Which
> leaves the caller with reference to already doomed struct file...
>
> IOW, NAK - this is completely broken. The whole point of
> atomic_long_add_unless() is that the check and conditional increment
> are atomic. Together. That's what your optimization takes out.
Cheers Al, yes, this is fscked.
As an aside, I've previously toyed with the idea of implementing a form
of cmpxchg() using a pair of xchg() operations and a smp_cond_load_relaxed(),
where the thing would transition through a "reserved value", which might
perform better with the current trend of building hardware that doesn't
handle CAS failure so well.
But I've never had the time/motivation to hack it up, and it relies on that
reserved value which obviously doesn't always work (so it would have to be a
separate API).
Will
Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
Hi Al,
在 2020/8/27 22:28, Al Viro 写道:
> On Thu, Aug 27, 2020 at 06:19:44PM +0800, Shaokun Zhang wrote:
>> From: Yuqi Jin
>>
>> It is well known that the performance of atomic_add is better than that of
>> atomic_cmpxchg.
>> The initial value of @f_count is 1. While @f_count is increased by 1 in
>> __fget_files, it will go through three phases: > 0, < 0, and = 0. When the
>> fixed value 0 is used as the condition for terminating the increase of 1,
>> only atomic_cmpxchg can be used. When we use < 0 as the condition for
>> stopping plus 1, we can use atomic_add to obtain better performance.
>
> Suppose another thread has just removed it from the descriptor table.
>
>> +static inline bool get_file_unless_negative(atomic_long_t *v, long a)
>> +{
>> +long c = atomic_long_read(v);
>> +
>> +if (c <= 0)
>> +return 0;
>
> Still 1. Now the other thread has gotten to dropping the last reference,
> decremented counter to zero and committed to freeing the struct file.
>
Apologies that I missed it.
>> +
>> +return atomic_long_add_return(a, v) - 1;
>
> ... and you increment that sucker back to 1. Sure, you return 0, so the
> caller does nothing to that struct file. Which includes undoing the
> changes to its refecount.
>
> In the meanwhile, the third thread does fget on the same descriptor,
> and there we end up bumping the refcount to 2 and succeeding. Which
> leaves the caller with reference to already doomed struct file...
>
> IOW, NAK - this is completely broken. The whole point of
> atomic_long_add_unless() is that the check and conditional increment
> are atomic. Together. That's what your optimization takes out.
>
How about this? We try to replace atomic_cmpxchg with atomic_add to improve
performance. The atomic_add does not check the current f_count value.
Therefore, the number of online CPUs is reserved to prevent multi-core
competition.
+
+static inline bool get_file_unless(atomic_long_t *v, long a)
+{
+ long cpus = num_online_cpus();
+ long c = atomic_long_read(v);
+ long ret;
+
+ if (c > cpus || c < -cpus)
+ ret = atomic_long_add_return(a, v) - a;
+ else
+ ret = atomic_long_add_unless(v, a, 0);
+
+ return ret;
+}
+
#define get_file_rcu_many(x, cnt) \
- atomic_long_add_unless(&(x)->f_count, (cnt), 0)
+ get_file_unless(&(x)->f_count, (cnt))
Thanks,
Shaokun
> .
>
Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
On Mon, Aug 31, 2020 at 09:43:31AM +0800, Shaokun Zhang wrote: > How about this? We try to replace atomic_cmpxchg with atomic_add to improve > performance. The atomic_add does not check the current f_count value. > Therefore, the number of online CPUs is reserved to prevent multi-core > competition. No. Really, really - no. Not unless you can guarantee that process on another CPU won't lose its timeslice, ending up with more than one increment happening on the same CPU - done by different processes scheduled there, one after another. If you have some change of atomic_long_add_unless(), do it there. And get it past the arm64 folks. get_file_rcu() is nothing special in that respect *AND* it has to cope with any architecture out there. BTW, keep in mind that there's such thing as a KVM - race windows are much wider there, since a thread representing a guest CPU might lose its timeslice whenever the host feels like that. At which point you get a single instruction on a guest CPU taking longer than many thousands of instructions on another CPU of the same guest. AFAIK, arm64 does support KVM with SMP guests.
RE: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
From: Al Viro > Sent: 31 August 2020 04:21 > > On Mon, Aug 31, 2020 at 09:43:31AM +0800, Shaokun Zhang wrote: > > > How about this? We try to replace atomic_cmpxchg with atomic_add to improve > > performance. The atomic_add does not check the current f_count value. > > Therefore, the number of online CPUs is reserved to prevent multi-core > > competition. > > No. Really, really - no. Not unless you can guarantee that process on > another > CPU won't lose its timeslice, ending up with more than one increment > happening on > the same CPU - done by different processes scheduled there, one after another. > > If you have some change of atomic_long_add_unless(), do it there. And get it > past the arm64 folks. get_file_rcu() is nothing special in that respect *AND* > it has to cope with any architecture out there. > > BTW, keep in mind that there's such thing as a KVM - race windows are much > wider there, since a thread representing a guest CPU might lose its timeslice > whenever the host feels like that. At which point you get a single > instruction > on a guest CPU taking longer than many thousands of instructions on another > CPU of the same guest. The same thing can happen if a hardware interrupt occurs. Not only the delays for the interrupt itself, but all the softint processing that happens as well. That can take a long time - even milliseconds. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
