Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()

[Petr Mladek <pmladek@suse.com>]

On Wed 2023-01-04 17:25:08, Petr Mladek wrote:
> On Fri 2022-12-30 19:27:28, Zhen Lei wrote:
> > Function __module_address() can quickly return the pointer of the module
> > to which an address belongs. We do not need to traverse the symbols of all
> > modules to check whether each address in addrs[] is the start address of
> > the corresponding symbol, because register_fprobe_ips() will do this check
> > later.
> > 
> > Assuming that there are m modules, each module has n symbols on average,
> > and the number of addresses 'addrs_cnt' is abbreviated as K. Then the time
> > complexity of the original method is O(K * log(K)) + O(m * n * log(K)),
> > and the time complexity of current method is O(K * (log(m) + M)), M <= m.
> > (m * n * log(K)) / (K * m) ==> n / log2(K). Even if n is 10 and K is 128,
> > the ratio is still greater than 1. Therefore, the new method will
> > generally have better performance.
> > 
> > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> > ---
> >  kernel/trace/bpf_trace.c | 101 ++++++++++++++++-----------------------
> >  1 file changed, 40 insertions(+), 61 deletions(-)
> > 
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 5f3be4bc16403a5..0ff9037098bd241 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2684,69 +2684,55 @@ static void symbols_swap_r(void *a, void *b, int size, const void *priv)
> >  	}
> >  }
> >  
> > -struct module_addr_args {
> > -	unsigned long *addrs;
> > -	u32 addrs_cnt;
> > -	struct module **mods;
> > -	int mods_cnt;
> > -	int mods_cap;
> > -};
> > -
> > -static int module_callback(void *data, const char *name,
> > -			   struct module *mod, unsigned long addr)
> > +static int get_modules_for_addrs(struct module ***out_mods, unsigned long *addrs, u32 addrs_cnt)
> >  {
> > -	struct module_addr_args *args = data;
> > -	struct module **mods;
> > -
> > -	/* We iterate all modules symbols and for each we:
> > -	 * - search for it in provided addresses array
> > -	 * - if found we check if we already have the module pointer stored
> > -	 *   (we iterate modules sequentially, so we can check just the last
> > -	 *   module pointer)
> > -	 * - take module reference and store it
> > -	 */
> > -	if (!bsearch(&addr, args->addrs, args->addrs_cnt, sizeof(addr),
> > -		       bpf_kprobe_multi_addrs_cmp))
> > -		return 0;
> > +	int i, j, err;
> > +	int mods_cnt = 0;
> > +	int mods_cap = 0;
> > +	struct module *mod;
> > +	struct module **mods = NULL;
> >  
> > -	if (args->mods && args->mods[args->mods_cnt - 1] == mod)
> > -		return 0;
> > +	for (i = 0; i < addrs_cnt; i++) {
> > +		mod = __module_address(addrs[i]);
> 
> This must be called under module_mutex to make sure that the module
> would not disappear.
> 
> > +		if (!mod)
> > +			continue;
> >  
> > -	if (args->mods_cnt == args->mods_cap) {
> > -		args->mods_cap = max(16, args->mods_cap * 3 / 2);
> > -		mods = krealloc_array(args->mods, args->mods_cap, sizeof(*mods), GFP_KERNEL);
> > -		if (!mods)
> > -			return -ENOMEM;
> > -		args->mods = mods;
> > -	}
> > +		/* check if we already have the module pointer stored */
> > +		for (j = 0; j < mods_cnt; j++) {
> > +			if (mods[j] == mod)
> > +				break;
> > +		}
> 
> This might get optimized like the original code.
> 
> My understanding is that the addresses are sorted in "addrs" array.
> So, the address is either part of the last found module or it belongs
> to a completely new module.

I thought more about it and I think that I was wrong, see below.

> 	for (i = 0; i < addrs_cnt; i++) {
> 		/*
> 		 * The adresses are sorted. The adress either belongs
> 		 * to the last found module or a new one.
> 		 *
> 		 * This is safe because we already have reference
> 		 * on the found modules.
> 		 */
> 		 if (mods_cnt && within_module(addrs[i], mods[mods_cnt - 1]))
> 			continue;

within_module() checks two sections (init and core). They are
allocated separately, see module_alloc() called in move_module().

There might be a section from another modules between the init
and core section of a module.

The optimization worked in the original code because
module_kallsyms_on_each_symbol() always iterated over all
symbols from a module.

That said, I am not sure if bpf trace might be added for
symbols in the module init section. But it might be
better to stay on the safe side.

Best Regards,
Petr