On Wed, Dec 03, 2025 at 02:46:22PM +0900, Harry Yoo wrote: > On Tue, Dec 02, 2025 at 05:00:08PM +0800, Hao Li wrote: > > Introduce barn_get_full_sheaf(), a helper that detaches a full sheaf from > > the per-node barn without requiring an empty sheaf in exchange. > > > > Use this helper in __pcs_replace_empty_main() to change how an empty main > > per-CPU sheaf is handled: > > > > - If pcs->spare is NULL and pcs->main is empty, first try to obtain a > > full sheaf from the barn via barn_get_full_sheaf(). On success, park > > the empty main sheaf in pcs->spare and install the full sheaf as the > > new pcs->main. > > > > - If pcs->spare already exists and has objects, keep the existing > > behavior of simply swapping pcs->main and pcs->spare. > > > > - Only when both pcs->main and pcs->spare are empty do we fall back to > > barn_replace_empty_sheaf() and trade the empty main sheaf into the > > barn in exchange for a full one. > > Hi Hao, > > Yeah this is a very subtle difference between __pcs_replace_full_main() > and __pcs_replace_empty_main(), that the former installs the full main > sheaf in pcs->spare, while the latter replaces the empty main sheaf with > a full sheaf from the barn without populating pcs->spare.
Exactly. > > Is it intentional, Vlastimil? > > > This makes the empty-main path more symmetric with > > __pcs_replace_full_main(), > > which for a full main sheaf parks the full sheaf in pcs->spare and pulls an > > empty sheaf from the barn. It also matches the documented design more > > closely: > > > > "When both percpu sheaves are found empty during an allocation, an empty > > sheaf may be replaced with a full one from the per-node barn." > > I'm not convinced that this change is worthwhile by adding more code; > you probably need to make a stronger argument for why it should be done. Hi Harry, Let me explain my intuition in more detail. Previously, when pcs->main was empty and pcs->spare was NULL, we used barn_replace_empty_sheaf() to trade the empty main sheaf into the barn in exchange for a full one. As a result, pcs->main became full, but pcs->spare remained NULL. Later, when frees filled pcs->main again, __pcs_replace_full_main() had to call into the barn to obtain an empty sheaf, because there was still no local spare to use. With this patch, when pcs->main is empty and pcs->spare is NULL, __pcs_replace_empty_main() instead uses barn_get_full_sheaf() to pull a full sheaf from the barn while keeping the now‑empty main sheaf locally as pcs->spare. The next time pcs->main becomes full, __pcs_replace_full_main() can simply swap main and spare, with no barn operations and no need to allocate a new empty sheaf. In other words, although we still need one barn operation when main first becomes empty in __pcs_replace_empty_main(), we avoid a future barn operation on the subsequent “main full” path in __pcs_replace_full_main. Thanks. > > > Signed-off-by: Hao Li <[email protected]> > > --- > > -- > Cheers, > Harry / Hyeonggon
