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Date: Fri, 15 Feb 2013 17:39:38 -0600
From: Nathan Zimmer <nzimmer@....com>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: <viro@...iv.linux.org.uk>, <eric.dumazet@...il.com>,
<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
David Woodhouse <dwmw2@...radead.org>,
Alexey Dobriyan <adobriyan@...il.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 resend] procfs: Improve Scaling in proc
On 02/15/2013 04:12 PM, Andrew Morton wrote:
> On Fri, 15 Feb 2013 14:47:54 -0600
> Nathan Zimmer <nzimmer@....com> wrote:
>
>> I am currently tracking a hotlock reported by a customer on a large system,
>> 512 cores. I am currently running 3.8-rc7 but the issue looks like it has been
>> this way for a very long time.
>> The offending lock is proc_dir_entry->pde_unload_lock.
>>
>> This patch converts the replaces the lock with the rcu. However the pde_openers
>> list still is controlled by a spin lock. I tested on a 4096 machine and the lock
>> doesn't seem hot at least according to perf.
>>
>> This is a refresh/resend of what was orignally suggested by Eric Dumazet some
>> time ago.
>>
>> Supporting numbers, lower is better, they are from the test I posted earlier.
>> cpuinfo baseline Rcu
>> tasks read-sec read-sec
>> 1 0.0141 0.0141
>> 2 0.0140 0.0142
>> 4 0.0140 0.0141
>> 8 0.0145 0.0140
>> 16 0.0553 0.0168
>> 32 0.1688 0.0549
>> 64 0.5017 0.1690
>> 128 1.7005 0.5038
>> 256 5.2513 2.0804
>> 512 8.0529 3.0162
>>
>> ...
>>
>> diff --git a/fs/proc/generic.c b/fs/proc/generic.c
>> index 76ddae8..6896a70 100644
>> --- a/fs/proc/generic.c
>> +++ b/fs/proc/generic.c
>> @@ -191,13 +191,16 @@ proc_file_read(struct file *file, char __user *buf, size_t nbytes,
>> struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
>> ssize_t rv = -EIO;
>>
>> - spin_lock(&pde->pde_unload_lock);
>> - if (!pde->proc_fops) {
>> - spin_unlock(&pde->pde_unload_lock);
>> + const struct file_operations *fops;
> There's now a stray newline in the definitions section.
Noted and corrected, in a few places.
>> + rcu_read_lock();
>> + fops = rcu_dereference(pde->proc_fops);
>> + if (!fops) {
>> + rcu_read_unlock();
>> return rv;
>> }
>> - pde->pde_users++;
>> - spin_unlock(&pde->pde_unload_lock);
>> + atomic_inc(&pde->pde_users);
>> + rcu_read_unlock();
> So what's up with pde_users? Seems that it's atomic_t *and* uses a
> form of RCU protection. We can't make it a plain old integer because
> it's modified under rcu_read_lock() and we can't move the atomic_inc()
> outside rcu_read_lock() because of the synchronization games in
> remove_proc_entry()?
The intent of pde_users is to let us know when it is safe to clean out
the pde_openers.
I probably should comment this.
>> rv = __proc_file_read(file, buf, nbytes, ppos);
>>
>>
>> ...
>>
>> @@ -802,37 +809,30 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
>> return;
>> }
>>
>> - spin_lock(&de->pde_unload_lock);
>> /*
>> * Stop accepting new callers into module. If you're
>> * dynamically allocating ->proc_fops, save a pointer somewhere.
>> */
>> - de->proc_fops = NULL;
>> - /* Wait until all existing callers into module are done. */
>> - if (de->pde_users > 0) {
>> - DECLARE_COMPLETION_ONSTACK(c);
>> -
>> - if (!de->pde_unload_completion)
>> - de->pde_unload_completion = &c;
>>
>> - spin_unlock(&de->pde_unload_lock);
>> + rcu_assign_pointer(de->proc_fops, NULL);
>> + synchronize_rcu();
>> + /* Wait until all existing callers into module are done. */
>>
>> + DECLARE_COMPLETION_ONSTACK(c);
> This should have generated a c99-style definition warning. Did your
> compiler version not do this?
A clear over site on my part.
>> + de->pde_unload_completion = &c;
>> + if (!atomic_dec_and_test(&de->pde_users))
>> wait_for_completion(de->pde_unload_completion);
>>
>> - spin_lock(&de->pde_unload_lock);
>> - }
>> -
>> + spin_lock(&de->pde_openers_lock);
>> while (!list_empty(&de->pde_openers)) {
>> struct pde_opener *pdeo;
>>
>> pdeo = list_first_entry(&de->pde_openers, struct pde_opener, lh);
>> list_del(&pdeo->lh);
>> - spin_unlock(&de->pde_unload_lock);
>> pdeo->release(pdeo->inode, pdeo->file);
>> kfree(pdeo);
>> - spin_lock(&de->pde_unload_lock);
>> }
>> - spin_unlock(&de->pde_unload_lock);
>> + spin_unlock(&de->pde_openers_lock);
>>
>> if (S_ISDIR(de->mode))
>> parent->nlink--;
>>
>> ...
>>
>> static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
>> {
>> + const struct file_operations *fops;
>> struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
>> loff_t rv = -EINVAL;
>> loff_t (*llseek)(struct file *, loff_t, int);
>>
>> - spin_lock(&pde->pde_unload_lock);
>> + rcu_read_lock();
>> + fops = rcu_dereference(pde->proc_fops);
>> /*
>> * remove_proc_entry() is going to delete PDE (as part of module
>> * cleanup sequence). No new callers into module allowed.
>> */
>> - if (!pde->proc_fops) {
>> - spin_unlock(&pde->pde_unload_lock);
>> + if (!fops) {
>> + rcu_read_unlock();
>> return rv;
>> }
>> /*
>> * Bump refcount so that remove_proc_entry will wail for ->llseek to
>> * complete.
>> */
>> - pde->pde_users++;
>> + atomic_inc(&pde->pde_users);
>> /*
>> * Save function pointer under lock, to protect against ->proc_fops
>> * NULL'ifying right after ->pde_unload_lock is dropped.
>> */
> This comment needs updating.
>
> However, it doesn't appear to be true any more. With this patch we no
> longer set ->fops to NULL in remove_proc_entry(). (What replaced that
> logic?)
>
> So are all these games with local variable `llseek' still needed?
> afaict the increment of pde_users will stabilize ->fops?
We still are setting de->proc_fops to NULL to prevent new callers.
Also we still have to save fops-> since we cannot use fops outside the
rcu_read_un/lock.
Unless I misunderstood your question.
But yes the comment needs to be updated.
>
>> - llseek = pde->proc_fops->llseek;
>> - spin_unlock(&pde->pde_unload_lock);
>> + llseek = fops->llseek;
>> + rcu_read_unlock();
>>
>> if (!llseek)
>> llseek = default_llseek;
>> @@ -182,15 +176,17 @@ static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count,
>> struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
>> ssize_t rv = -EIO;
>> ssize_t (*read)(struct file *, char __user *, size_t, loff_t *);
>> + const struct file_operations *fops;
>>
>> - spin_lock(&pde->pde_unload_lock);
>> - if (!pde->proc_fops) {
>> - spin_unlock(&pde->pde_unload_lock);
>> + rcu_read_lock();
>> + fops = rcu_dereference(pde->proc_fops);
>> + if (!fops) {
>> + rcu_read_unlock();
>> return rv;
>> }
>> - pde->pde_users++;
>> - read = pde->proc_fops->read;
>> - spin_unlock(&pde->pde_unload_lock);
>> + atomic_inc(&pde->pde_users);
>> + read = fops->read;
>> + rcu_read_unlock();
> Many dittoes.
>
>> if (read)
>> rv = read(file, buf, count, ppos);
>> @@ -204,15 +200,17 @@ static ssize_t proc_reg_write(struct file *file, const char __user *buf, size_t
>> struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
>> ssize_t rv = -EIO;
>> ssize_t (*write)(struct file *, const char __user *, size_t, loff_t *);
>> + const struct file_operations *fops;
>>
>> - spin_lock(&pde->pde_unload_lock);
>> - if (!pde->proc_fops) {
>> - spin_unlock(&pde->pde_unload_lock);
>> + rcu_read_lock();
>> + fops = rcu_dereference(pde->proc_fops);
>> + if (!fops) {
>> + rcu_read_unlock();
>> return rv;
>> }
>> - pde->pde_users++;
>> - write = pde->proc_fops->write;
>> - spin_unlock(&pde->pde_unload_lock);
>> + atomic_inc(&pde->pde_users);
>> + write = fops->write;
>> + rcu_read_unlock();
>>
>> ...
>>
Thanks,
Nate
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