Re: [PATCH v4 0/5] mm: zone lock tracepoint instrumentation

[Matthew Wilcox <willy@infradead.org>]

On Mon, Mar 09, 2026 at 03:13:17PM -0400, Steven Rostedt wrote:
> The biggest issue with making a generic light weight LOCK_STAT is that
> locks are extremely optimized. Any addition of generic lock encoding will
> cause a noticeable overhead when compiled in, even when disabled.

I'm not sure that's true.  Taking the current Debian kernel config
leads to a "call" instruction to acquire a spinlock:

void __insert_inode_hash(struct inode *inode, unsigned long hashval)
{
        struct hlist_head *b = inode_hashtable + hash(inode->i_sb, hashval);

        spin_lock(&inode_hash_lock);
        spin_lock(&inode->i_lock);
        hlist_add_head_rcu(&inode->i_hash, b);
        spin_unlock(&inode->i_lock);
        spin_unlock(&inode_hash_lock);
}

compiles to:

[...]
     280:       23 35 00 00 00 00       and    0x0(%rip),%esi        # 286 <__insert_inode_hash+0x56>
                        282: R_X86_64_PC32      .data..ro_after_init+0x10
     286:       48 8d 2c f0             lea    (%rax,%rsi,8),%rbp
     28a:       e8 00 00 00 00          call   28f <__insert_inode_hash+0x5f>
                        28b: R_X86_64_PLT32     _raw_spin_lock-0x4
     28f:       4c 89 e7                mov    %r12,%rdi
     292:       e8 00 00 00 00          call   297 <__insert_inode_hash+0x67>
                        293: R_X86_64_PLT32     _raw_spin_lock-0x4
[...]

Debian doesn't do anything too weird here:

#
# Lock Debugging (spinlocks, mutexes, etc...)
#
CONFIG_LOCK_DEBUGGING_SUPPORT=y
# CONFIG_PROVE_LOCKING is not set
# CONFIG_LOCK_STAT is not set
# CONFIG_DEBUG_RT_MUTEXES is not set
# CONFIG_DEBUG_SPINLOCK is not set
# CONFIG_DEBUG_MUTEXES is not set
# CONFIG_DEBUG_WW_MUTEX_SLOWPATH is not set
# CONFIG_DEBUG_RWSEMS is not set
# CONFIG_DEBUG_LOCK_ALLOC is not set
# CONFIG_DEBUG_ATOMIC_SLEEP is not set
# CONFIG_DEBUG_LOCKING_API_SELFTESTS is not set
# CONFIG_LOCK_TORTURE_TEST is not set
# CONFIG_WW_MUTEX_SELFTEST is not set
# CONFIG_SCF_TORTURE_TEST is not set
# CONFIG_CSD_LOCK_WAIT_DEBUG is not set

(The spinlock code is too complex for me to follow what config options
influence whether it's a function call; you probably have enough of it
in your head that you'd know)

> The other issue is the data we store for the lock. A lock is usually just a
> word (or long) in size, embedded in a structure. LOCKDEP and LOCK_STAT adds
> a key per lock. This increases the data size of the kernel.

It does, but perhaps for a light weight lockstat, we could do better
than that.  For example it could use the return address to look up
which lock is being accessed rather than embedding a key in each lock.