Re: [RFC v1 05/14] fs: Move call_rcu() to call_rcu_lazy() in some paths

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, May 12, 2022 at 05:07:17PM -0700, Paul E. McKenney wrote:
> On Thu, May 12, 2022 at 03:04:33AM +0000, Joel Fernandes (Google) wrote:
> > This is required to prevent callbacks triggering RCU machinery too
> > quickly and too often, which adds more power to the system.
> > 
> > When testing, we found that these paths were invoked often when the
> > system is not doing anything (screen is ON but otherwise idle).
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> Some of these callbacks do additional work.  I don't immediately see a
> problem, but careful and thorough testing is required.

Full agreed, fwiw I did choose several of them by looking at what the
callback did but I agree additional review and testing is needed.

thanks,

 - Joel


> 
> 							Thanx, Paul
> 
> > ---
> >  fs/dcache.c     | 4 ++--
> >  fs/eventpoll.c  | 2 +-
> >  fs/file_table.c | 3 ++-
> >  fs/inode.c      | 2 +-
> >  4 files changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index c84269c6e8bf..517e02cde103 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -366,7 +366,7 @@ static void dentry_free(struct dentry *dentry)
> >  	if (unlikely(dname_external(dentry))) {
> >  		struct external_name *p = external_name(dentry);
> >  		if (likely(atomic_dec_and_test(&p->u.count))) {
> > -			call_rcu(&dentry->d_u.d_rcu, __d_free_external);
> > +			call_rcu_lazy(&dentry->d_u.d_rcu, __d_free_external);
> >  			return;
> >  		}
> >  	}
> > @@ -374,7 +374,7 @@ static void dentry_free(struct dentry *dentry)
> >  	if (dentry->d_flags & DCACHE_NORCU)
> >  		__d_free(&dentry->d_u.d_rcu);
> >  	else
> > -		call_rcu(&dentry->d_u.d_rcu, __d_free);
> > +		call_rcu_lazy(&dentry->d_u.d_rcu, __d_free);
> >  }
> >  
> >  /*
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index e2daa940ebce..10a24cca2cff 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -728,7 +728,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
> >  	 * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
> >  	 * use of the rbn field.
> >  	 */
> > -	call_rcu(&epi->rcu, epi_rcu_free);
> > +	call_rcu_lazy(&epi->rcu, epi_rcu_free);
> >  
> >  	percpu_counter_dec(&ep->user->epoll_watches);
> >  
> > diff --git a/fs/file_table.c b/fs/file_table.c
> > index 7d2e692b66a9..415815d3ef80 100644
> > --- a/fs/file_table.c
> > +++ b/fs/file_table.c
> > @@ -56,7 +56,8 @@ static inline void file_free(struct file *f)
> >  	security_file_free(f);
> >  	if (!(f->f_mode & FMODE_NOACCOUNT))
> >  		percpu_counter_dec(&nr_files);
> > -	call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
> > +
> > +	call_rcu_lazy(&f->f_u.fu_rcuhead, file_free_rcu);
> >  }
> >  
> >  /*
> > diff --git a/fs/inode.c b/fs/inode.c
> > index 63324df6fa27..b288a5bef4c7 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -312,7 +312,7 @@ static void destroy_inode(struct inode *inode)
> >  			return;
> >  	}
> >  	inode->free_inode = ops->free_inode;
> > -	call_rcu(&inode->i_rcu, i_callback);
> > +	call_rcu_lazy(&inode->i_rcu, i_callback);
> >  }
> >  
> >  /**
> > -- 
> > 2.36.0.550.gb090851708-goog
> >